Ruoff and co-workers recently demonstrated low-temperature (1193 K) homoepitaxial diamond growth from liquid gallium solvent. To develop an atomistic mechanism for diamond growth underlying this remarkable demonstration, we carried out density functional theory-based molecular dynamics (DFT-MD) simulations to examine the mechanism of single-crystal diamond growth on various low-index crystallographic diamond surfaces (100), (110), and (111) in liquid Ga with CH4. We find that carbon linear chains form in liquid Ga and then react with the growing diamond surface, leading first to the formation of carbon rings on the surface and then initiation of diamond growth. Our simulations find faster growth on the (110) surface than on the (100) or (111) surfaces, suggesting the (110) surface as a plausible growth surface in liquid Ga. For (110) surface growth, we predict the optimum growth temperature to be ∼1300 K, arising from a balance between the kinetics of forming carbon chains dissolved in Ga and the stability of carbon rings on the growing surface. We find that the rate-determining step for diamond growth is dehydrogenation of the growing hydrogenated (110) surface of diamond. Inspired by the recent experimental studies by Ruoff and co-workers demonstrating that Si accelerates diamond growth in Ga, we show that addition of Si into liquid Ga significantly increases the rate of dehydrogenating the growing surface. Extrapolating from the DFT-MD predicted rates at 2800 to 3500 K, we predict the growth rate at the experimental growth temperature of 1193 K, leading to rates in reasonable agreement with the experiment. These fundamental mechanisms should provide guidance in optimizing low-temperature diamond growth.

Calculated proton affinities (PAs) and gas phase basicities (GPBs) are reported for diamantane (C14H20), triamantane (C18H24), 'globular and planar' isomers of tetramantane (C22H28) and pentamantane (C26H32), and for one 'globular' isomer of each of the larger diamondoid molecules: C51H58, C78H72, C102H90, and C131H116. Assuming CxHy as the parent diamondoid molecule, we calculated PA and GPB values for a variety of CxHy+1+ isomers, as well as for the reaction CxHy + H+ yielding CxHy-1+ + H2(g); the latter is slightly favored based on GPB values for diamantane through pentamantane, but less favored compared to certain CxHy+1+ isomers of C51H58, C102H90, and C131H116. Indeed, the GPB values of C51H58, C102H90, and C131H116 classifiy them as 'superbases'. Calculations that had the initial location of the proton in an interstitial site inside the diamondoid molecule always showed the H having moved to the outside of the diamondoid molecule; for this reason, we focused on testing a variety of initial configurations with the proton placed in an initial position on the surface. Additional protons were added to determine the limiting number that could be, per these calculations, taken up by the diamondoid molecules and the maximum number of protons are shown in parentheses: C14H20(2), C18H24(3), C22H28(3), C26H32(3), C51H58(4). Bader charge distributions obtained for CxHy+1+ isomers (for diamantane through pentamantane) suggest that the positive charge is essentially completely delocalized over all the H atoms. NMR spectra were calculated for different isomers of C14H19+, and compared to the published NMR spectrum for when diamantane was mixed with magic acid and H2(g) was produced.

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.

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.

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.

To date, thousands of publications have reported chemical vapor deposition growth of "single layer" graphene, but none of them has described truly single layer graphene over large area because a fraction of the area has adlayers. It is found that the amount of subsurface carbon (leading to additional nuclei) in Cu foils directly correlates with the extent of adlayer growth. Annealing in hydrogen gas atmosphere depletes the subsurface carbon in the Cu foil. Adlayer-free single crystal and polycrystalline single layer graphene films are grown on Cu(111) and polycrystalline Cu foils containing no subsurface carbon, respectively. This single crystal graphene contains parallel, centimeter-long ≈100 nm wide "folds," separated by 20 to 50 µm, while folds (and wrinkles) are distributed quasi-randomly in the polycrystalline graphene film. High-performance field-effect transistors are readily fabricated in the large regions between adjacent parallel folds in the adlayer-free single crystal graphene film.

Ab initio calculations have been performed on the liquid gallium–hydrogen system at 100 °C. Gallium was found to interact with both free hydrogen atoms and H2, transferring charge in the process. F...

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.

First-principles density functional theory calculations were performed on a porphyrin-like motif into the lattice of carbon nanotubes and graphene. The porphyrin-like motif was generated by applying the Stone-Thrower-Wales (STW) transformation twice on two consecutive carbon bonds in a semiconducting (10 0) single-walled carbon nanotube (SWCNT) and graphene, resulting in a porphyrin-like motif that contained an octagon surrounded by four pentagons, two hexagons, and two heptagons. When one carbon atom of each pentagon is substituted by nitrogen (N-pyrrolic doping), the motif mimics the skeleton of a porphyrin molecule (DSTW-N4-porphyrin-like motif). Transition metals (TMs) (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) are incorporated to the double Stone-Thrower-Wales (DSTW)-N4-porphyrin motif. The band-structure, electronic density of states, binding energy, formation energy, and wave functions were calculated. The binding and formation energy calculations demonstrated that the proposed TM-DSTW-N4 defects are stable and energetically competitive with other types of defects. The calculated systems exhibit spin-dependent semiconducting band gap and half-metallicity. Our investigations offered insights into how TM atoms are adsorbed by sp2 carbon materials doped with N-pyrrolic. The interplay between the type of nitrogen doping (pyridine, substitutional, and pyrrolic) and structural defects in sp2 carbon materials are crucial for tailoring the electronic, magnetic, and catalytic properties.

The extended line defect (ELD) mimicking grain boundaries in two-dimensional systems is theoretically investigated in BN, GaN, and AlN semiconductor materials with a single layer honeycomb structure. The ELD consists of octagonal-square membered rings. Density functional calculations of the electronic density of states, scanning tunneling microscopy and transmission electron microscopy image simulations are analyzed. Our results revealed that the ELDs are stable in all considered monolayers. In addition, electronic density of states calculations demonstrated that in gap states are emerged when ELD is incorporated into the honeycomb structures. Finally, results on armchair nanoribbons with bare-edges and hydrogenated edges are discussed. (C) 2016 Elsevier B.V. All rights reserved.

BAs and AlN are semiconductor materials with an indirect and direct gap respectively in the bulk phase. Recently, electronic calculations have demonstrated that a single-layer or few layers of BAs and AlN exhibit a graphite-like structure with interesting electronic properties. In this work, infinite sheets single-layer heterojunction structures based on alternated strips with honeycomb BAs and AlN layers are investigated using first-principles density functional theory calculations. Optimized geometries, density of states, band-gaps, formation energies, and wave functions are studied for different strip widths joined along zigzag and armchair edges. Results in optimized heterojunction geometries revealed that BAs narrow strips exhibit a corrugation effect due to a lattice mismatch. It was found that zigzag heterojunctions are more energetically favored than armchair heterojunctions. Furthermore, the formation energy presents a maximum at the point where the heterojunction becomes a planar structure. Electronic charge density results yielded a more ionic behavior in Al-N bonds than the B-As bonds in accordance with monolayer results. It was observed that the conduction band minimum for both heterojunctions exhibit confined states located mainly at the entire AlN strips whereas the valence band maximum exhibits confined states located mainly at BAs strips. We expect that the present investigation will motivate more experimental and theoretical studies on new layered materials made of III-V semiconductors. (C) 2016 Elsevier B.V. All rights reserved.

Nowadays, III-V semiconductors are interesting candidate materials for the tailoring of two dimensional (2D) graphene-like structures. These new 2D materials have attracted profound interest opening the possibility to find semiconductor materials with unexplored properties. First-principles density functional theory calculations are performed in order to investigate the electronic properties of GaN planar and nanotube morphologies based on Haeckelite structures (containing octagonal and square membered rings). Optimized geometries, band-structures, phonon dispersion, binding energies, transmission electron microscopy images simulations, x-ray diffraction patterns, charge densities, and electronic band gaps are calculated. We demonstrated that GaN Haeckelite structures are stable exhibiting a semiconducting behavior with an indirect band gap. Furthermore, it was found that GaN Haeckelite nanotubes are semiconductor with a band gap nature (direct or indirect) that depends of the nanotube's chirality and diameter. In addition, it was demonstrated that surface passivation and the interaction with hydrazine, water, ammonia, and carbon monoxide molecules can change the band-gap nature. Our results are compared with the corresponding GaN hexagonal honeycomb structures.

We compute the structural and electronic properties of 111 -oriented InAs / InP nanowire heterostructures using Keating’s valence force field and a tight-binding model. We focus on the optical properties exciton energies and polarization of InAs quantum dots embedded in InP nanowires and on the height of InP and InAsP tunnel barriers embedded in InAs nanowires. We show that InAs quantum dots exhibit bright optical transitions, at variance with the highly mismatched InAs /GaAs nanowire heterostructures. The polarization of the photons is perpendicular to the nanowire for thin InAs layers but rotates parallel to the nanowire for thick enough ones, as a result of the increasing light-hole character of the exciton. As for tunnel barriers, we show that the residual strains can significantly reduce the conduction band discontinuity in thin InAsP layers. This must be taken into account in the design of nanowire tunneling devices.