Metal Carbide Films Research Articles

Bottom-Up Growth of Monolayer Honeycomb SiC.

The long theorized two-dimensional allotrope of SiC has remained elusive amid the exploration of graphenelike honeycomb structured monolayers. It is anticipated to possess a large direct band gap (2.5eV), ambient stability, and chemical versatility. While sp^{2} bonding between silicon and carbon is energetically favorable, only disordered nanoflakes have been reported to date. Here we demonstrate large-area, bottom-up synthesis of monocrystalline, epitaxial monolayer honeycomb SiC atop ultrathin transition metal carbide films on SiC substrates. We find the 2D phase of SiC to be almost planar and stable at high temperatures, up to 1200 °C in vacuum. Interactions between the 2D-SiC and the transition metal carbide surface result in a Dirac-like feature in the electronic band structure, which in the case of a TaC substrate is strongly spin-split. Our findings represent the first step towards routine and tailored synthesis of 2D-SiC monolayers, and this novel heteroepitaxial system may find diverse applications ranging from photovoltaics to topological superconductivity.

Influence of deposition temperature and amorphous carbon on microstructure and oxidation resistance of magnetron sputtered nanocomposite CrC films

Abstract It is known that mechanical and tribological properties of transition metal carbide films can be tailored by adding an amorphous carbon (a-C) phase, thus making them nanocomposites. This paper addresses deposition, microstructure, and for the first time oxidation resistance of magnetron sputtered nanocomposite Cr C/a-C films with emphasis on studies of both phases. By varying the deposition temperature between 20 and 700 °C and alternating the film composition, it was possible to deposit amorphous, nanocomposite, and crystalline Cr C films containing about 70% C and 30% Cr, or 40% C and 60% Cr. The films deposited at temperatures below 300 °C were X-ray amorphous and 500 °C was required to grow crystalline phases. Chronoamperometric polarization at +0.6 V vs. Ag/AgCl (sat. KCl) in hot 1 mM H2SO4 resulted in oxidation of Cr C, yielding Cr2O3 and C, as well as oxidation of C. The oxidation resistance is shown to depend on the deposition temperature and the presence of the a-C phase. Physical characterization of film surfaces show that very thin C/Cr2O3/Cr C layers develop on the present material, which can be used to improve the oxidation resistance of, e.g. stainless steel electrodes.

Impact of strain on the surface properties of transition metal carbide films: First-principles study

The effect of in-plane lattice strain on the atomic and electronic properties of low-index transition metal (M=Ti, Nb, and Ta) carbide surfaces is studied by first-principles molecular dynamics calculations using a pseudopotential plane-wave technique. The most stable cubic rock-salt phase is considered for carbides. The first-principle study of various [(001), (110), and metal-terminated (111)] carbide surfaces reveals that both compressive and tensile strains strongly affect surface relaxation and electronic properties (work function values and band structures). The most stable (001) carbide surfaces exhibit rumpling between transition metal and carbon atoms in the topmost surface layers, which depends on the applied strain. The work function (WF) for the metal-terminated (111) surfaces varies monotonically, rather strongly depending on the applied strain (the range of variation reaches about 1 eV), while the WF for the (001) surface varies nonmonotonically with a much smaller resulting variation over the wide range of the applied strains. Surface energy calculations show that surface stability is also governed by the applied strain.

Deposition of epitaxial transition metal carbide films and superlattices by simultaneous direct current metal magnetron sputtering and C60 evaporation

Thin epitaxial TiC and VC films and superlattices have been deposited on MgO(001) by simultaneous sputtering of the metals and evaporation of C60. It was found that epitaxial growth conditions for TiC could be maintained down to a temperature of 100 °C, while the epitaxial growth of VC required 200 °C. Epitaxial VC films were completely relaxed at all growth temperatures, while a change from a relaxed to a strained growth behavior was observed for TiC films. The structural quality of the TiC films was better than for the VC films. A general observation was that a plasma-assisted deposition process yields films with a higher quality and allows epitaxial growth at lower temperatures than for a pure coevaporation process.

Surface properties of metal-nitride and metal-carbide films deposited on Nb for radio-frequency superconductivity

Various effects occur which can prevent attainment of the high Q’s and/or the high gradient fields necessary for the operation of radio-frequency (rf) superconducting cavities. One of these effects, multipactor, both causes the cavity to detune during filling due to resonant secondary electron emission at the cavity walls, and lowers the quality factor (Q) by dissipative processes. TiN deposited onto the high-field regions of room-temperature Al cavities has been used at the Stanford Linear Accelerator Center to successfully reduce multipactor in the past. We have therefore studied TiN and its companion materials, NbN, NbC, and TiC, all on Nb substrates under several realistic conditions: (1) as-deposited, (2) exposed to air, and (3) electron bombarded. The studied films (up to 14-nm thickness) were sputter deposited onto sputter-cleaned Nb substrates. Results indicate that all the materials tested gave substantially the same results. The maximum secondary electron yields for as-deposited films were reduced to nearly the preoxidized values after electron bombardment (2–3×1017 electrons cm−2 in the case of NbN and NbC). X-ray photoelectron spectroscopy analysis showed that the oxides (e.g., TiO2 in the case of TiN films) formed during air exposure were slightly reduced (converted to lower oxides) by the electron-beam exposure. Auger electron spectroscopy (AES) showed a slight reduction in the surface O concentration following beam exposure. These results suggest that the chemical nature of top surface layers is responsible for the substantial changes in secondary electron yield observed upon electron-beam exposures and that AES does not reflect this change strongly because of the difficulty in extracting chemical (versus elemental) information from AES. The results indicate that any of these films would be poor choices if simply deposited and exposed to air, but, in fact, the in situ electron bombardment which occurs during cavity operation serves to reduce the secondary electron yield and thereby causes a substantial reduction in multipacting.

SELECTIVE PROPERTIES OF ROUGH SPUTTERED FILMS

Solar selective surfaces based on textured copper have been produced by means of two sputtering techniques. In a novel planar magnetron cosputtering system sputter etching of bulk copper sheet, when seeded by a flux of titanium atoms, led to the formation of microscopically roughened surfaces with unusually complex structural formations. Preliminary sputter etching experiments have also been carried out in a cylindrical magnetron system. Alternatively, a standard planar magnetron system was used to deposit copper films of thickness 0.5 µm to 10.0 µm onto glass at sputtering pressures in the range 0.5 Pa to 100 Pa. This deposition technique also resulted in the formation of microscopically roughened surfaces, in this case characterised by arrays of cones. Optical properties of both types of surface follow well defined trends as deposition conditions vary. High absorptance, low emittance selective surfaces have been produced by coating homogeneous metal carbide films onto suitably textured copper on glass films. This composite selective surface exhibits good stability at temperatures up to 500°C in vacuum.

Magnetron-sputtered metal carbide solar selective absorbing surfaces

A range of magnetron reactively sputtered metal carbide interference filters has been deposited onto sputtered copper on glass substrates and aged in vacuum at elevated temperature. These films show considerable promise as selective surfaces for incorporation in all glass vacuum insulated solar energy collectors. Surfaces produced by depositing homogeneous metal carbide films have solar absorptances α∠80% and emittances ε∠2.0% at 300 K. Solar absorptances above 90% and emittances 3.0%–4.0% at 300 K are obtained for multilayer metal carbide films fabricated by varying the reactive gas flow rate in steps during the deposition. Ageing experiments show that the reflectances of the multilayer films alter due to interdiffusion between the layers, resulting in a slow degradation of solar absorptance. Some diffusion coefficients for carbon in stainless steel and titanium films have been evaluated. Surfaces fabricated using chromium, molybdenum, and stainless steel carbides are superior to titanium carbide surfaces. Magnetron sputtering technology offers the possibility of more rapid and efficient deposition of these selective surfaces compared with the high pressure diode deposition technique utilized in previous work.

Sputtered metal carbide solar-selective absorbing surfaces

A range of sputtered metal carbide interference filters has been prepared on copper substrates. These show promise as high-temperature-stable selective surfaces for solar energy photothermal conversion. Solar absorptances of 75%–80% and room temperature emittances of about 2% can be obtained at 300 K for single layer metal carbide films on bulk copper. The emittance of an iron carbide film on copper increased to 7% at 600°C. The solar absorptance of the same film was temperature independent up to 500°C. Absorptances of 80%–90% and room temperature emittances of about 4% can be obtained for two-layer interference filters on copper. The coating of both glass and metal surfaces is discussed.