The preparation of high quality oriented diamond thin films via low temperature and hydrogen ion etched nucleation

A three-step process was used to prepare high quality [001]-oriented diamond films. First, diamond crystallites were nucleated for 20 min on Si(001) at a temperature around 740 °C by bias-enhance method, during this step the portion of [001]-oriented diamond nuclei was increased in comparison with the nuclei deposited by a two-step method. Then hydrogen ion etching was performed for 30 min by setting an electric potential of −140 V. After the etching step most of the crystallites were [001]-oriented and twinned crystallites disappeared. Finally, diamond thin films were deposited under conventional conditions for [001]-textured growth. SEM was used to analyse the morphology of diamond crystallites and films. The results indicate that large area, uniform and [001]-oriented diamond thin films can be prepared by three-step growth. The films show good Raman characteristics and higher thermal conductivity than those deposited by a two-step process.     

Preferential {1 0 0} etching of boron-doped diamond electrodes and diamond particles by CO2 activation

The etching behavior of polycrystalline boron-doped diamond (BDD) electrodes and diamond particles with gaseous CO₂ at 800 and 900 °C was investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. Polycrystalline BDD (800 ppm), composed of a mixture of cubic {1 0 0} and triangular {1 1 1} orientated planes, was used so as to pursue the possibility of preferential etching by high temperature CO₂ treatment. Nanometer sized pits were observed on the {1 0 0} planes while no change was observable for the {1 1 1} planes when the activation temperature was 800 °C. The difference in the etching behavior by CO₂ with regard to the different planes was clarified using diamond particles and comparing with steam activation. The results demonstrate that CO₂ activation leads to preferential {1 0 0} etching, whereas steam-activation results in preferential {1 1 1} etching.     

Low-energy excitations of graphene on Ru(0 0 0 1)

The structure and the acoustic phonon branches of graphene on Ru(0 0 0 1) have been experimentally investigated with helium atom scattering (HAS) and analyzed by means of density functional theory (DFT) including Grimme dispersion forces. In-plane interactions are unaffected by the interaction with the substrate. The energy of 16 meV for the vertical rigid vibration of graphene against the Ru(0 0 0 1) surface layer indicates an interlayer effective force constant about five times larger than in graphite. The Rayleigh mode observed for graphene/Ru(0 0 0 1) is almost identical to the one measured on clean Ru(0 0 0 1). This is accounted for by the strong bonding to the substrate, which also explains the previously reported high reflectivity to He atoms of this system. Finally, we report the observation of an additional acoustic branch, closely corresponding to the one already observed by HAS in graphite, which cannot be ascribed to any phonon mode and suggests a possible plasmonic origin.     

Structural and optical characterizations of nanostructured diamond films grown on Si(0 0 1) substrates

Nanostructured diamond films (NDFs) were grown on fine-polished Si(0 0 1) substrates by radio-frequency plasma-enhanced hot-filament chemical vapor deposition at low gas pressure of approximately 2 Torr. High resolution field-emission scanning electron microscopy (FESEM), X-ray diffraction and Raman scattering spectroscopy were employed to characterize the NDFs. FESEM measurement indicated that diamond nuclei density exceeded 10¹⁰ cm⁻² after growth for 20 min, which should render potential applications, such as in the fabrication of diamond-related field-emission devices. Photoluminescence excitation characterizations revealed the sharp optical absorption band edge of NDFs, and the band gap is 5.34 eV.     

Epitaxial lateral overgrowth (ELO) of homoepitaxial diamond through an iridium mesh

In the present work iridium layers forming a mesh on diamond have been studied as potential candidates for buried electrodes or stopping layers in an ELO process for heteroepitaxial diamond. Thin iridium layers (∼ 15 nm) were deposited by e-beam evaporation at ∼ 700 °C on the facets of individual (001)-oriented CVD diamond crystallites and macroscopic Ib HPHT substrates with off-axis angles of several degrees. The heteroepitaxial iridium films formed a mesh with 10–200 nm large holes. These were penetrated by homoepitaxial diamond in a microwave plasma chemical vapour deposition process (MWPCVD) burying the iridium layer completely after 15 min of diamond growth. High resolution X-ray diffraction including reciprocal space mapping and Raman spectroscopy was used to characterize the structural properties of the diamond overlayer on the Ib HPHT substrate. It was monocrystalline with an FWHM of 0.03–0.05° of the X-ray rocking curve. Its lattice planes were tilted by ∼ 0.01° with respect to the substrate and showed a macroscopic strain of − 10^− 4 perpendicular to the surface. Besides the smaller lattice constant due to the lack of nitrogen the strain is mostly attributed to a tensile in-plane stress state. Strain and tilt can be attributed to the lateral overgrowth and the off-axis angle of the substrate.     

Catalytic conversion of olefins on supported cubic platinum nanoparticles: Selectivity of (1 0 0) versus (1 1 1) surfaces

The surface chemistry and catalytic conversion of cis- and trans-2-butenes on platinum (1 0 0) facets were characterized via surface-science and catalytic experiments. Temperature-programed desorption studies on Pt(1 0 0) single crystals pointed to the higher hydrogenation probability of the trans isomer at the expense of a lower extent of CC double-bond isomerization. To test these trends under catalytic conditions, shape selective catalysts were prepared by dispersing cubic platinum colloidal nanoparticles (which expose only (1 0 0) facets) onto a high-surface-area silica xerogel support. Infrared absorption spectroscopy and transmission electron microscopy were used to determine the conditions needed to remove the organic surfactants without loosing the original narrow size distribution and cubic shape of the original metal nanoparticles. Catalytic kinetic measurements with these materials corroborated the surface-science predictions, and pointed to a switch in isomerization selectivity from preferential cis-to-trans conversion with Pt(1 0 0) surfaces to the reverse trans-to-cis reaction with Pt(1 1 1) facets.     

Revealing the atomic structure of the buffer layer between SiC(0 0 0 1) and epitaxial graphene

On the SiC(0 0 0 1) surface (the silicon face of SiC), epitaxial graphene is obtained by sublimation of Si from the substrate. The graphene film is separated from the bulk by a carbon-rich interface layer (hereafter called the buffer layer) which in part covalently binds to the substrate. Its structural and electronic properties are currently under debate. In the present work we report scanning tunneling microscopy (STM) studies of the buffer layer and of quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling the buffer layer from the SiC(0 0 0 1) substrate by means of hydrogen intercalation. Atomic resolution STM images of the buffer layer reveal that, within the periodic structural corrugation of this interfacial layer, the arrangement of atoms is topologically identical to that of graphene. After hydrogen intercalation, we show that the resulting QFMLG is relieved from the periodic corrugation and presents no detectable defect sites.     

Periodically rippled graphene on Ru(0 0 0 1): A template for site-selective adsorption of hydrogen dimers via water splitting and hydrogen-spillover at room temperature

The interaction of water with periodically rippled graphene deposited on Ru(0 0 0 1) and nearly-flat graphene/Pt(1 1 1) has been investigated by using high-resolution electron energy loss spectroscopy. Graphene samples were exposed to ambient air humidity as well as to water molecules under controlled conditions in vacuum. In both cases, loss measurements show that water molecules dosed at room temperature dissociate giving rise to C–H bonds. We suggest that water molecules intercalate under graphene and are split by the underlying metal catalyst. On the lattice-mismatched graphene/Ru(0 0 0 1) interface, the corrugation of the graphene overlayer induces site selectivity for H adsorption in ortho and para dimers. On the other hand, no dimer formation occurs for the nearly-flat, multidomain graphene/Pt(1 1 1) interface.     

Diamond growth on Ir/CaF2/Si substrates

A new multi-layered structure of heteroepitaxial (1 0 0) and (1 1 1) Ir grown on CaF₂-buffered (0 0 1) and (1 1 1) Si wafers by UHV electron-beam evaporation was prepared for the deposition of diamond films. A two-step process of bias-enhanced nucleation and a subsequent growth by controlling the α growth parameter was performed to deposit (0 0 1) and (1 1 1) diamond films by chemical vapor deposition, respectively. Scratching or seeding by fine diamond powders was also attempted on the (1 1 1) substrates to enhance the diamond nucleation density. Raman spectroscopy, X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy were used to characterize the Ir/CaF₂/Si substrates as well as the diamond films grown on top of iridium layer. Heteroepitaxial relationship between the deposited diamond grains and (0 0 1) substrates has been observed.     

Preparation of 4-inch Ir/YSZ/Si(001) substrates for the large-area deposition of single-crystal diamond

Diamond/Ir/YSZ/Si(001) is currently the most promising multilayer structure for the future realisation of large-area diamond single crystals. A decisive key is the preparation of the iridium layers on silicon. It is shown in this work that high quality iridium films with mosaic spread below 0.2° can be grown on oxide buffer layers with a mosaic spread higher than 1°. An averaging process during the coalescence of the iridium islands provides a plausible mechanism for this phenomenon. The oxide buffer and the iridium overlayers can be grown homogeneously on 4-inch wafers in a similar quality as for 1 × 1 cm² samples. Bias enhanced nucleation followed by 40 h growth on the large-area Ir/YSZ/Si(001) wafers yields diamond films with a mosaicity of 0.16° (tilt) and 0.34° (twist). For a further increase of the area of heteroepitaxial diamond nucleation the homogeneity of the plasma discharge has to be improved.     

Structural characterization of Ce1−xZrxO2 (0 ≤ x ≤ 1) samples prepared at 1650 °C by solid state reaction: A combined TEM and XRD study

In this paper we present the structural study of the Ce_1−xZr_xO₂ (0 ≤ x ≤ 1) samples, prepared at 1650 °C by solid state reaction. The study was performed by means of powder X-ray diffraction (XRD) and selected area electron diffraction (SAED) techniques. The Rietveld analysis of XRD patterns shows that all the samples adopt the fluorite structure although with different symmetries. In the Zr-rich area (x > 0.8) the monoclinic phase (S.G. P2₁/c) is detected, while in the Ce-rich area the cubic solid solution (S.G. Fm-3m) is stabilized in a wide compositional range (0 ≤ x ≤ 0.4). For x = 0.5, the metastable tetragonal T′ phase is detected as unique phase. In the intermediate region (0.6 ≤ x ≤ 0.8), XRD patterns are formed by those of the stable cubic and tetragonal T phases besides that of the metastable T′ phase (S.G. P4₂/nmc). Through the Rietveld analysis, sites occupation and relative percentages of different phases have been determined. Cation miscibility was investigated by means of selected area electron diffraction (SAED) combined with EDS microanalysis at the nanoscale level. Solubility ranges deduced are discussed in terms of phase diagrams previously reported.     

785 nm Raman spectroscopy of CVD diamond films

Raman spectroscopy is a powerful technique often used to study CVD diamond films, however, very little work has been reported for the Raman study of CVD diamond films using near-infrared (785 nm) excitation. Here, we report that when using 785 nm excitation with 1 µm spot size, the Raman spectra from thin polycrystalline diamond films exhibit a multitude of peaks (over 30) ranging from 400–3000 cm^− 1. These features are too sharp to be photoluminescence, and are a function of film thickness. For films > 30 µm thick, freestanding films, and for films grown in diamond substrates the Raman peaks disappear. This suggests that the laser is probing the vibrations of molecular units at the grain boundaries of the disordered crystallites present at the interface between the diamond and substrate.     

Transmission electron microscopy study of the very early stages of diamond growth on iridium

The diamond nuclei generated during bias enhanced nucleation (BEN) on iridium were not detected so far by high resolution transmission electron microscopy (HRTEM). The aim of the present work was to investigate their earliest appearance after BEN by applying very short growth steps, ranging from 5 s to 1 min. On all the samples with growth step crystalline diamond could be identified unequivocally by HRTEM and reflection high energy electron diffraction (RHEED). After 5 s the former nuclei have evolved into crystallites of 2 nm thickness and about 10 nm width. At that time the carbon precursor phase which appears amorphous in HRTEM and which was formed by the ion bombardment has largely disappeared. After 10 s no residues are left, which proves that the 1–2 nm-thick amorphous carbon layer is only stable under biasing conditions. The rapid etching of the precursor phase and the simultaneous slow increase in volume of the tiny diamond crystals results in a minimum in carbon coverage several seconds after termination of the BEN process.     

Formation of high-quality quasi-free-standing bilayer graphene on SiC(0 0 0 1) by oxygen intercalation upon annealing in air

We report on the conversion of epitaxial monolayer graphene on SiC(0 0 0 1) into decoupled bilayer graphene by performing an annealing step in air. We prove by Raman scattering and photoemission experiments that it has structural and electronic properties that characterize its quasi-free-standing nature. The (6√3 × 6√3)R30° buffer layer underneath the monolayer graphene loses its covalent bonding to the substrate and is converted into a graphene layer due to the oxidation of the SiC surface. The oxygen reacts with the SiC surface without inducing defects in the topmost carbon layers. The high-quality bilayer graphene obtained after air annealing is p-doped and homogeneous over a large area.     

New high Q microwave dielectric ceramics with rock salt structures: (1 − x)Li2TiO3 + xMgO system (0 ≤ x ≤ 0.5)

The structural evolution and microwave dielectric properties of (1 ? x)Li₂TiO₃ + xMgO system (0 ≤ x ≤ 0.5) have been investigated in this paper. The ordering degree decreased with the increase of MgO content. The microcracks and cleavage on (0 0 1) due to the weak Li–O bonds disappeared with the increase of MgO content. The dielectric constant and temperature coefficient of resonant frequency decreased with the increase of MgO content. The Q × f value increased with x up to x = 0.2 and then decreases with the further increase of x. An excellent combined microwave dielectric properties could be obtained when x = 0.24, ?_r = 19.2, Q × f = 106,226 GHz and τ_f = 3.56 ppm/°C.     

Two routes for HFCVD diamond nucleation on Si(1 1 1) studied by HRTEM

The early stages of diamond nucleation on Si(1 1 1) thinned areas have been studied by several TEM techniques. Both top-view and cross-section investigations have been carried out to better characterise the nature of the diamond precursor as well as the diamond/silicon interface. Two different routes for diamond nucleation have been underlined using the same CVD experimental conditions. One involves the formation of a β-SiC interlayer while the second is a direct nucleation on silicon. Moreover, the size of the smallest detectable diamond particles is a few nanometers according to nanodiffraction experiments.     

Density functional theory study on adsorption of thiophene on TiO2 anatase (0 0 1) surfaces

In order to develop a fundamental understanding of the adsorption mechanism of thiophenic compounds on TiO₂-based adsorbents for ultra-deep desulfurization of liquid hydrocarbon fuels, a density functional theory (DFT) study was conducted on the adsorption of thiophene over the TiO₂ anatase (0 0 1) surface. The perfect, O-poor (with oxygen vacancies), and O-rich (with activated O₂ on the surface) anatase (0 0 1) surfaces were built, and the interaction of thiophene molecule with these surfaces was examined. The adsorption configuration and adsorption energy on the different surfaces and sites were estimated. The results showed that thiophene may be adsorbed on both the perfect and O-poor surfaces through an interaction between the Ti cations on the surface and the S atom in thiophene, whereas on the O-rich surface through an interaction of the activated O atoms (the dissociatively or associatively adsorbed O₂) on the surface with the S atom in thiophene to form a sulfone-like surface species. The adsorption of thiophene on the O-rich surface is significantly stronger than adsorption on the perfect and O-poor surfaces on the basis of the calculated adsorption energies. The results indicate that the activated O₂ on the TiO₂ anatase (0 0 1) surface may play an important role in the adsorption desulfurization over the TiO₂-based adsorbents, and increased concentration of the activated O₂ on the surface may result in improvement of the adsorption capacity of the adsorbents.     

Ethylene conversion to ethylidyne on Pd(1 1 1) and Pt(1 1 1): A first-principles-based kinetic Monte Carlo study

We present kinetic Monte Carlo simulations of ethylene conversion to ethylidyne on Pd(1 1 1) and Pt(1 1 1) surfaces, on the basis of reaction enthalpies and barriers obtained from periodic density functional calculations. We considered three possible mechanisms encompassing four different intermediates, ethyl, vinyl, ethylidene, and vinylidene. Our simulations predict that the most plausible pathway on both surfaces is ethylene → vinyl → vinylidene → ethylidyne. In contrast to earlier suggestions that the dehydrogenation to vinyl is rate-limiting on Pt(1 1 1), we found the hydrogenation of vinylidene to ethylidyne to be crucial on this surface. On Pd(1 1 1), the initial dehydrogenation of ethylene is rate-limiting. Hence, vinylidene species accumulate on Pt(1 1 1), while all intermediates on Pd(1 1 1) convert rapidly to ethylidyne without accumulation. The simulated apparent activation energies for the formation of ethylidyne on Pd(1 1 1), 94 kJ mol^?1, and on Pt(1 1 1), 65 kJ mol^?1, agree well with experimental results.     

Preparation of (0 2 0)-oriented BaTi2O5 thick films and their dielectric responses

Barium dititanate (BaTi₂O₅) thick films were prepared on a Pt-coated Si substrate by laser chemical vapor deposition, and ac electric responses of (0 2 0)-oriented BaTi₂O₅ films were investigated using several equivalent electric circuit models. BaTi₂O₅ films in a single phase were obtained at a Ti/Ba molar ratio (m_Ti/Ba) of 1.72–1.74 and deposition temperature (T_dep) of 908–1065 K as well as m_Ti/Ba = 1.95 and T_dep = 914–953 K. (0 2 0)-oriented BaTi₂O₅ films were obtained at m_Ti/Ba = 1.72–1.74 and T_dep = 989–1051 K. BaTi₂O₅ films had columnar grains, and the deposition rate reached 93 μm h^?1. The maximum relative permittivity of the (0 2 0)-oriented BaTi₂O₅ film prepared at T_dep = 989 K was 653 at 759 K. The model of an equivalent circuit involving a parallel combination of a resistor, a capacitor, and a constant phase element well fitted the frequency dependence of the interrelated ac electrical responses of the impedance, electric modulus, and admittance of (0 2 0)-oriented BaTi₂O₅ films.