ta-C Surface Research Articles

Growth of self-assembled monolayer on tetrahedral amorphous carbon film coated magnetic head

The magnetic heads overcoated by tetrahedral amorphous carbon (ta-C) films have been immersed in the 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) solvent at different predetermined dipping time. The growth of FDTS self-assembled monolayer (SAM) on the ta-C surface has been confirmed by time-of-flight secondary ion mass spectrometry. Four distinct stages were observed in the growing process: (1) low coverage (nucleation), (2) growing up (3) coalescence and (4) complete monolayer, similar to the theoretical models of epitaxial film growth at low temperatures. Contact angle measurement using de-ionized water is carried out to study the time dependence of contact angle during the growth process of FDTS monolayer. The surface morphology, profiles and roughness (rms) of FDTS monolayer were determined by atomic force microscopy (AFM). Furthermore, the variation of monolayer thickness and distribution was revealed by transmission electron microscopy, which is in agreement with the AFM and contact angle results.

Nonlinear effects in collision cascades and high energy shock waves during ta-C:H growth

The surface topography of hydrogenated tetrahedral amorphous carbon (ta-C:H) is critical for various applications such as microelectromechanical devices, magnetic and optical storage devices, and medical implants. The surface topography of ta-C:H films deposited by distributed electron cyclotron resonance plasma from C2H2 gas precursor was investigated. The effects of pressure, together with ion flux and energy, are studied by atomic force microscopy in relation to the structural evolution of the films. The results are compared with the predictions of the Edward-Wilkinson model [Proc. R. Soc. London, Ser. A 44, 1039 (1966)] recently proposed to account for ta-C:H growth and with previous interpretations based on hypersonic shock waves. The random hillocks observed on the smooth surfaces of ta-C:H films deposited at high pressure are thought to result from the interference of high energy shock waves triggered by C4Hx+ ions that produce overlapping collision cascades and induce nonlinear effects.

Morphological change of carbon surfaces by sputter erosion

In order to study the allotropic effect on ripple formation on carbon surfaces induced by sputter erosion, three types of carbon materials, highly oriented pyrolytic graphite (HOPG), single crystalline diamond and tetrahedral amorphous carbon (ta-C) films, were irradiated with 5-keV Xe+ at an incident angle of 30°. The irradiation fluence was 2×1017cm−2 for all irradiations. No ripples were formed on diamond and ta-C surfaces, while ripples with a wavelength of ∼100nm occurred on HOPG. Electron energy loss spectroscopy revealed that the surfaces of all carbon materials became amorphous with similar densities upon irradiation. Therefore, the presented results clearly show that rippling depends on the nature of the underlying carbon materials.

Fabrication of high-aspect-ratio microscale Ta mold inserts with micro electrical discharge machining

We report fabrication of microscale Ta mold inserts by micro-electrical-discharge-machining (μEDM). Morphology, chemistry, and structure of the near-surface region of as-μEDMed Ta blanks have been characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. A TaC surface layer forms on as-μEDMed Ta surfaces. This altered surface layer was removed by electro-chemical-polishing. Further modification of Ta insert surfaces was accomplished by deposition of a conformal Ti-containing hydrogenated carbon coating. We demonstrate successful replication of high-aspect-ratio microscale structures in Al and Cu by compression molding with such surface-engineered Ta mold inserts.

Trends in the chemical properties of early transition metal carbide surfaces: A density functional study

In this paper we present density functional theory (DFT) investigations of the physical, chemical and electronic structure properties of several close-packed surfaces of early transition metal carbides, including β-Mo2C(0001), and the (111) surfaces of TiC, VC, NbC, and TaC. The results are in excellent agreement with experimental values of lattice constants and bulk moduli. The adsorption of atomic hydrogen is used as a probe to compare the chemical properties of various carbide surfaces. Hydrogen adsorbs more strongly to the metal-terminated carbide surfaces than to the corresponding closest-packed pure metal surfaces, due to the tensile strain induced in the carbide surfaces upon incorporation of carbon into the lattice. Hydrogen atoms were found to adsorb more weakly on carbide surfaces than on the corresponding closest-packed pure metal surfaces only when there were surface carbon atoms present, and in some cases very stable C–H species were formed. The DFT results indicated that the hydrogen adsorption energy could be correlated to the d-band center of the carbide surfaces, although the correlation was not as good as our previous studies on bimetallic surfaces.

Structural investigation and gas barrier performance of diamond-like carbon based films on polymer substrates

In this report, tetrahedral amorphous carbon (ta-C), hydrogenated amorphous carbon (a-C:H), silicon doped tetrahedral amorphous carbon (ta-C:Si:H), and silicon doped hydrogenated amorphous carbon (a-C:H:Si) films with thickness in the range 50–370 nm have been produced by PECVD (Plasma Enhanced Chemical Vapour Deposition) and FCVA ( Filtered Cathodic Vacuum Arc) techniques on Polyethylene terepthalate (PET) and polycarbonate (PC) substrates. The paper is concerned with exploring the links between the atomic structure, gas barrier performance in carbon based films deposited on polymer substrates. A range of techniques including XRR, NEXAFS, Raman, surface profilometry, nano-indentation and water vapour permeation analysis were used to analyze the microstructure and properties of the films. The intensity and area of π* peak at the C K (carbon) edge of the NEXAFS spectra was lower in the FCVA films in comparison to that of PECVD ones confirming the higher sp 3 content of FCVA films. The surface of ta-C films showed a network of micro-cracks, which is detrimental for gas barrier application. However, the surfaces of both ta-C:H:Si and a-C:H:Si silicon-incorporated films were almost free of cracks. We also found that the incorporation of Si into both types of DLC films lead to a significant reduction of water vapour transmission rate.

Friction and wear at nanometer scale: a comparative study of hard carbon films

Tribological properties of nanostructured carbon (ns-C) and tetrahedral amorphous carbon (ta-C) thin films were investigated by friction force microscopy. It was found that the ns-C films have a smaller friction coefficient than ta-C films for relative humidity greater than 30%. In particular, at 40% of humidity, ns-C films have lower friction coefficient (0.11±0.02) than the ta-C films (0.13±0.02), which can be attributed to both the presence of closed graphite nanoparticles and the passivation of the dangling bonds at the ns-C surface. The friction coefficient did not vary as a function of the tip scanning velocity for both films. The nanoscale wear was studied in a very low force regime, in the range of nanonewton, using an atomic force microscope (AFM) with a Si 3N 4 tip and with forces in the range of micronewton with the AFM equipped with a stainless still cantilever and a diamond tip. The ns-C provides better wear resistance compared to ta-C films in the range of forces studied. The sp 2-rich ta-C surface layer was easily scratched during the wear test in contrast to the ns-C films. The wear in ta-C in the low forces regime is attributed to the presence of this low density layer at the surface of the film due to subplantation of energetic ions during deposition while the better resistance to wear of ns-C films is attributed to its highly elastic nature.

Self-assembled monolayers on ta-C surfaces: Effect of sp3/sp2 ratio on adsorption rate and friction

Self-assembled monolayers of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), CF3(CF2)7(CH2)2SiCl3 were deposited on tetrahedral amorphous carbon (ta-C) surfaces by immersion process. The ta-C films were deposited on silicon substrates by using a filtered cathodic arc system and the sp3/sp2 ratios of the ta-C films were controlled by changing the sample bias voltage. In results of the x-ray photoelectron spectroscopy measurements, it was found that the adsorption rate of silane molecules on sample A (whose sp3 content is about 85%) was higher than that of on sample B (whose sp3 content is about 65%). This trend was qualitatively the same as that found for water contact angle measurement. The friction forces of FDTS/sample A and FDTS/sample B measured by lateral force microscope show FDTS/sample A has lower friction value than that of FDTS/sample B. We conclude the following: the density of dangling bonds, which are effective adsorption sites for silane molecules, increases with sp3 content. Consequently, the adsorption rate of silane molecules increases with increasing sp3 content. The friction force decreases with increasing sp3 content of the ta-C film since more of a close-packed self-assembled monolayer can be formed.

Ab initiodensity functional studies of stepped TaC surfaces

We report on density functional total energy calculations of the step formation and interaction energies for vicinal TaC(001) surfaces. Our calculations show that double and triple-height steps are favored over single-height steps for a given vicinal orientation, which is in agreement with recent experimental observations. We provide a description of steps in terms of atomic displacements and charge localization and predict an experimentally observable rumpled structure of the step-edges, where the Ta atoms undergo larger displacements compared to the C atoms.

The surface stoichiometry of VC 0.80(100) studied by LEED

The surface structure of substoichiometric vanadium carbide was studied by quantitative low-energy electron diffraction on a VC 0.80(100) sample. A simple 1 × 1 diffractogram was observed on annealing to 1000°C. Optimization in four- to seven-dimensional structural spaces gave two surface structures, close to one another with respect to the agreement between theoretical and experimental spectra but widely separated by the structure and composition of the topmost VC layer. In the primary (secondary) model the top layer is rippled with the C subplane protruding 0.88 ± 0.05 A ̊ ( 0.20 ± 0.15 A ̊ ) from the V subplane and with a C occupancy 0.4 ± 0.2 (0.2 ± 0.2). The primary model is supported by a previous photo-emission study of VC 0.80(100), where a strong 1s core level signal from the surface C atoms was observed. Characteristic of VC 0.80(100) is the expansion of the first interplanar spacing of the C sublattice, while for the (100) surfaces of TaC, HfC and VN 0.89 a contraction of the first interplanar spacing of the metal sublattice predominates.

Scanning-tunneling-microscopy study of faceting on high-step-density TaC surfaces.

We have studied the morphologies of the TaC(310), (210), and (110) surfaces using scanning tunneling microscopy. Heating the crystals to high temperatures activates a faceting of these surfaces into a hill-and-valley structure consisting of enlarged (100) terraces and (010) step walls. Step-separation distributions obtained from these surfaces can be well fit by sharp Gaussians and are much narrower than predicted for the noninteracting terrace-step-kink model, indicating a strong repulsive interaction exists between steps on the faceted surfaces. This faceting is suggested to be driven by a decrease in the total step repulsive energy through a reduction of the total number of step pairs. \textcopyright{} 1996 The American Physical Society.

Mean-square displacements of Ta and C atoms in TaC encapsulated in carbon nanotubes

X-ray diffraction measurements for stoichiometric TaC single crystals encapsulated in carbon nanotubes in the temperature range from 7 to 273 K are reported. Mean-square displacements of Ta and C atoms, 〈u2〉, in the encapsulated crystals are derived from the integrated intensities of the (hkl) Bragg reflections by assuming 〈uTa2〉=〈uC2〉. The experimental value of √〈u2〉 at 273 K is in agreement with the rippled displacements of the bulk TaC (100) surface atoms, which have been explored in a low energy electron diffraction measurement. The effective Debye temperature Θ′ derived from its temperature dependence is 340±10 K, which is about 70% of that for bulk TaC. The predicted softening of transverse acoustic surface phonon mode (the Rayleigh mode) along the M̄ symmetry axis in the surface Brillouin zone, by Ishida and Terakura, is in good accord with the decrease of Θ′. Linear thermal expansivities, Δa/a273, for the encapsulated crystals and for bulky crystalline particles show a marked difference below 150 K, where a273 is the lattice constant of TaC at 273 K. The predicted surface phonon mode (the Rayleigh mode), with a small anomaly, is adequate for explaining the starting temperature of the observed difference.

Interaction of hydrogen with TaC(111) and NbC(111) surfaces: Angle-resolved photoemission study.

Interaction of hydrogen with TaC(111) and NbC(111) surfaces: Angle-resolved photoemission study.

Change in the electronic states of graphite overlayers depending on thickness.

Electronic states of graphite overlayers formed on the TaC(111) surface have been investigated with the use of scanning tunneling microscopy and photoelectron spectroscopy. The graphite film grows on the substrate layer by layer. The thickness of the overlayer has been adjusted precisely to be either one or two monolayers. The physical properties of the monolayer graphite film are modified by chemical bonding at the interface. This interfacial bonding becomes weak upon the formation of the second layer of graphite, which makes the properties of the double-layer graphite film similar to those of bulk graphite.

TaC(110): a periodic facet reconstruction studied by LEED and STM

A simple model is established to describe diffraction phenomena (LEED) from a unidirectionally reconstructed TaC(110) surface. By comparing diffraction data with results of detailed simulations, the reconstruction is found to consist of alternate (100) and (010) facets propagating periodically along the [110̄] direction like an irregular ridge-and-valley grating, with (110) terraces completely absent. The irregularity is due to variation in facet sizes, and the mean separation is determined to be L = 6 lattice spacings. The validity of this model is supported by STM imaging results which confirm the structure obtained from the LEED-data analysis.

Linear-muffin-tin-orbital calculation of TaC(001) surface relaxation.

Recent experimental work on the (001) surface of tantalum carbide has shown that this material is among the class of compounds which exhibit a marked rippling relaxation. We report here on a total-energy calculation of the relaxation of TaC(001), using a full-potential, linear-combination-of-muffin-tin-orbitals electronic-structure method. We obtain good agreement between calculated and experimental values for the degree of relaxation, and a detailed analysis of the physical mechanisms involved leads us to conclude that the nature of the rippled relaxation is determined primarily by the electrostatics associated with the bulk charge density. We also discuss other properties of the TaC(001) surface, including surface core-level shifts.

Electronic structure of monolayer graphite on some transition metal carbide surfaces

It has been revealed that monolayer graphite on the (111) surfaces of transition-metal carbides differs substantially from bulk graphite in the lattice constant and the phonon dispersion, while those of monolayer graphite on the (100) surfaces are hardly modified. In this work, the electronic structure of monolayer graphite formed on TiC(111), (100), and TaC(100) surfaces has been investigated by XPS, UPS, and EELS. From the results of XPS and UPS, we have concluded that these large changes are caused not by charge transfer but mainly by orbital hybridization between the graphite overlayer and the (111) surface. This is in contrast to the case of graphite intercalation compounds. By angle-resolved EELS, in addition, we have clearly detected a two-dimensional character of the plasmons in monolayer graphite.

Periodic faceting on TaC(110): Observations using high-resolution low-energy electron diffraction and scanning tunneling microscopy.

Unidirectional alternating (100)-(010) faceting has been observed for the clean TaC(110) surface using both high-resolution low-energy electron diffraction and scanning tunneling microscopy. Facets propagate periodically along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction and form a ridge-and-valley grating, characterized by an average periodicity of \ensuremath{\sim}6 lattice spacings in the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction, where (110) terraces are completely absent. This faceted surface is shown to be an energetically favored structure.

Terrace-width and step-height enlargement: Reconstruction of the TaC(310) surface.

The geometric structure of the TaC(310) surface has been investigated using high-resolution low-energy electron diffraction. An ideally truncated TaC(310) surface has a staircase geometry where each unit consists of a (100) terrace, three row spacings wide, separated by a single-height step. However, it is determined that the surface is reconstructed with both terrace width and step height tripled on average while the surface still retains the (310) orientation. In addition, a small number of double-height steps are found to coexist with the triple-height steps. Possible physical origins of these observations are discussed.

Oxygen chemisorption on transition-metal carbide (100) surfaces studied by X-ray photoelectron spectroscopy and low-energy He + scattering

The mechanism for oxidation of the (100) surfaces of TiC, VC, ZrC, and TaC has been studied using X-ray photoemission and low-energy He + ion scattering. The oxidation of TiC(100) is found to proceed into the bulk even at room temperature by replacing a part of the surface carbon atoms with oxygen and subsequent diffusion of oxygen into the bulk via carbon vacancies suggesting that the preferential bonding of oxygen with carbon exists at the surface. In case of the (100) surfaces of VC and TaC oxidation into the bulk is prevented by formation of an additional metal-oxygen bond at the surface. These significant differences will be elucidated on the basis of the electronic band structures characteristic of the transition-metal carbides.