Amorphous Carbon Surfaces Research Articles

Cross-sectional TEM study of surface modification of nano-structure with gas cluster ion beams

ABSTRACTSurface modification effects on patterned surface with gas cluster ion beam (GCIB) were studied by observation with a cross-sectional transmission electron microscope in order to use it for planarization of patterned media such as discrete track media (DTM) or bit-patterned media (BPM) for future hard disk drives. As a model structure of patterned media, line-and-space or bit patterns were fabricated on Si substrates, and subsequently amorphous carbon films were deposited on them. After Ar-GCIB irradiations on amorphous carbon, it was shown that GCIB preferentially removed bumps or crest on the surface of amorphous carbon at normal incidence. The required thickness for planarization was close to the initial peak-to-valley. At an incident angle of 57°, line-and-space patterns became sharp-pointed shape. On the contrary, line-and-space patterns were planarized without tiny asperity formation at 77°. These results indicate that quite effective planarization of patterned surface is possible using GCIB at normal or glancing angle irradiation.

Thermal stability of perfluorinated molecular monolayers immobilized on pulsed laser deposited amorphous carbon surfaces

Amorphous carbon (a-C) films grown by Pulsed Laser Deposition were optimized to get smooth surfaces with sp3-rich hybridization, sp3/(sp2+sp3) = 0.45-0.65. Under UHV annealing conditions, some sp3-to-sp2 conversion mechanism becomes efficient above 350°C. Covalent immobilization of linear alkene molecular layers was performed by a thermally-assisted (160-300°C) process, using perfluorodecene (PFD) in the gas phase, with no physical or chemical surface preparation of a-C films. Quantitative XPS analysis indicates the immobilization of a dense molecular monolayer. Thermal stability of the grafted PFD monolayer is investigated by using UHV annealing steps to derive the desorption rate constant k(T) and the temperature corresponding to half initial coverage (T1/2 = 460°C). The slower desorption rate of perfluorodecene monolayers immobilized on a-C as compared with a-Si:H surfaces reveals the role of the covalent C-C interface bond in the robustness of the molecule / semiconductor assembly.

Facile abstraction of hydrogen atoms from graphane, diamond, and amorphous carbon surfaces: A first-principles study

ion of surface hydrogen by atomic H from graphane, diamond(001), diamond(111), and hydrogenated amorphous carbon (a-C:H) surfaces was studied using density-functional theory calculations in the generalized gradient approximation. Our calculations show that for each surface, the abstraction reaction is highly exothermic with a negligible activation energy barrier. The degree of exothermicity depends on the type of surface and on the local bonding environment of the site from which the H atom was abstracted. Detailed analyses of the reactions and of atomic charge densities along reaction pathways indicate a direct-impact Eley-Rideal mechanism for the abstractions.

Cesium adsorption induced inversion of the field electron emission from tungsten covered with nanostructural carbon

It has been established that the field electron emission from an amorphous carbon film containing graphite nanoclusters exhibits inversion upon the adsorption of cesium atoms on the sample surface. The phenomenon consists in the disappearance of electron emission from graphite nanoclusters, which were the local emission centers prior to the metal adsorption, and the appearance of homogeneous emission from the amorphous carbon surface not emitting before that. The observed effect is explained using notions of the surface diffusion of cesium in an inhomogeneous electric field, cesium intercalation into graphite nanoclusters, and the related reversal of the dipole moment upon a change in the electric field direction.

Effects of tip geometry on interfacial contact forces

Experimental techniques that utilize atomic force microscopy are routinely used to examine tribological properties of tip–sample interactions. While analysis of data obtained with these methods provides values for macroscale properties, such as interfacial shear strength, understanding nanoscale properties, such as contact radius, requires an atomic-scale approach. Molecular dynamics simulations provide the ability to numerically analyze the nanoscale origins of a wide-range of material and tribological properties. In this paper, the sliding contact between a self-assembled monolayer (SAM) and two countersurfaces (a nominally flat, amorphous carbon surface and a nearly spherical fullerene tip) is compared. By examining contact forces between the tip and monolayer atoms, large differences in monolayer behavior that occur due to tip geometry can be elucidated. The structure factor reveals that the fullerene tip creates a more disordered monolayer than the amorphous counterface. Friction forces were also studied using the atomic-level contact forces, which show that the depth at which the fullerene tip affects the SAMs substrate is much deeper than the amorphous counterface. The distribution of contact forces that contribute to friction and load were studied and show a difference in behavior between the two countersurfaces. Finally, while there are a large number of atoms that have a non-zero load during sliding, a smaller subset of 32 atoms carries ∼96% of the load. Using this subset of atoms to compute contact radius reveals a greater agreement with the continuum mechanics models than using all atoms with a non-zero load. This paper highlights how computer simulations can yield insight into tribological interactions at the atomic scale.

Photochemical Grafting of Alkenes onto Carbon Surfaces: Identifying the Roles of Electrons and Holes

We report a mechanistic investigation of the charge transfer processes that occur during photochemical grafting of liquid alkenes to H-terminated surfaces of diamond and amorphous carbon. Spectrally resolved photoelectron yield experiments were performed to directly characterize the photoemission of electrons from the hydrogen-terminated surfaces into liquid alkenes, using trifluoroacetamide-protected 1-aminodec-1-ene (TFAAD) and 10-N-Boc-aminodec-1-ene (tBoc) as model alkenes having different terminal acceptor groups; 1-dodecene was also used as a control. Corresponding X-ray and ultraviolet photoelectron spectroscopy measurements (XPS, UPS) establish a clear correlation between the photoelectron yield, the grafting efficiency at different wavelengths, and the valence electronic structure of the substrate and of the reactant molecule. Direct imaging of the molecular layers via scanning electron microscopy shows that there are substantial differences in the sharpness of molecular patterns that can be prod...

Atomistic Factors Governing Adhesion between Diamond, Amorphous Carbon and Model Diamond Nanocomposite Surfaces

Complementary atomic force microscopy (AFM) measurements and molecular dynamics (MD) simulations were conducted to determine the work of adhesion for diamond (C)(111)(1 × 1) and C(001)(2 × 1) surfaces paired with carbon-based materials. While the works of adhesion from experiments and simulations are in reasonable agreement, some differences were identified. Experimentally, the work of adhesion between an amorphous carbon tip and individual C(001)(2 × 1)–H and C(111)(1 × 1)–H surfaces yielded adhesion values that were larger on the C(001)(2 × 1)–H surface. The simulations revealed that the average adhesion between self-mated C(001)(2 × 1) surfaces was smaller than for self-mated C(111)(1 × 1) contacts. Adhesion was reduced when amorphous carbon counterfaces were paired with both types of diamond surfaces. Pairing model diamond nanocomposite surfaces with the C(111)(1 × 1)–H sample resulted in even larger reductions in adhesion. These results point to the importance of atomic-scale roughness for adhesion. The simulated adhesion also shows a modest dependence on hydrogen coverage. Density functional theory calculations revealed small, C–H bond dipoles on both diamond samples, with the C(001)(2 × 1)–H surface having the larger dipole, but having a smaller dipole moment per unit area. Thus, charge separation at the surface is another possible source of the difference between the measured and calculated works of adhesion.

Evolution of carbon surfaces under simulated bombardment by deuterium

The bombardment of both graphite and deuterated amorphous carbon surfaces with 20 eV D atoms has been performed using molecular dynamics simulation. The primary purpose of these simulations is to determine whether the eventual state of the surface, once it has reached a steady state, is independent of the starting structure. It is found that while independently realized amorphous carbon structures give rise to similar impact-modified surfaces, the graphitic surface evolves towards a somewhat different structure. Including or neglecting a realistic treatment of the nonbonded interactions in the graphite bombardment does not result in large differences in the impact-modified structure, although the penetration depth is considerably less when nonbonded interactions are included.

Kinetic Monte Carlo method for simulating astrochemical kinetics: Test calculations of molecular hydrogen formation on interstellar dust particles

Molecular hydrogen formation on the surfaces of interstellar dust grains has been studied numerically. Different stochastic methods (analogue Monte Carlo methods, direct solutions of the chemical master equation, continuous-time random-walk simulations, etc.) have been recently applied to the astrochemistry of hydrogen formation; however, the computational efficiency of these approaches is usually low because they are extremely time consuming (Herbst and Shematovich, 2003). A kinetic version of the Monte Carlo method based on splitting by physical processes is presented in the study. Each of the basic physical processes—adsorption of atomic hydrogen on the surface, thermal diffusion, formation of molecular hydrogen, and hydrogen desorption from the surface-are considered as independent random Markovian-type processes and are simulated using stochastic algorithms. The suggested numerical model is computationally efficient allowing calculations to be held on model surfaces with up to 106 active centers, which corresponds to interstellar grains of up to 0.4 μm in diameter. Test calculations of the efficiency of molecular hydrogen formation were held for the two models of the surfaces of interplanetary grains, namely, for the homogeneous surfaces of olivine and amorphous carbon. It was confirmed that the effective formation of molecular hydrogen in diffuse molecular clouds is possible only within a narrow range of dust-particle temperatures. A comparison with the numerical results of other authors is presented.

Thermal grafting of organic monolayers on amorphous carbon and silicon (111) surfaces: A comparative study

Thermally-assisted (160 °C) liquid phase grafting of linear alkene molecules has been performed simultaneously on amorphous carbon (a-C) and hydrogen passivated crystalline silicon Si(111):H surfaces. Atomically flat a-C films with a high sp 3 average surface hybridization, sp 3 / (sp 2 + sp 3) = 0.62, were grown using pulsed laser deposition (PLD). Quantitative analysis of X-ray photoelectron spectroscopy, X-ray reflectometry and spectroscopic ellipsometry data show the immobilization of a densely packed (> 3 × 10 14 cm − 2) single layer of organic molecules. In contrast with crystalline Si(111):H and other forms of carbon films, no surface preparation is required for the thermal grafting of alkene molecules on PLD amorphous carbon. The molecular grafted a-C surface is stable against ambient oxidation, in contrast with the grafted crystalline silicon surface.

Z-Tetraol composition and bonding to the underlying carbon surface

The bonding kinetics for 10 Å Z-Tetraol films on the amorphous nitrogenated carbon surface (CNx) is investigated as a function of the Z-Tetraol molecular polarity. Z-Tetraol nominally contains 2 hydroxyl groups per chain end. The molecular polarity of Z-Tetraol is varied by the addition of a “bis-adduct” component of Z-Tetraol. This material contains 3 instead of 2 hydroxyl groups per chain end. The bonding kinetics and changes in the surface energy as a function of time in thin film mixtures is investigated under ambient conditions. The initial bonded fraction scales with the bis-adduct concentration and can be increased by as much as 50%. The asymptotic bonded fraction exhibits a more modest increase of approximately 10%.

Transfer of rovibrational energies in hydrogen plasma–carbon surface interactions

We present state-of-the-art molecular dynamics (MD) simulations of high-density plasma-bombardment of carbon, mimicking plasma–wall interactions at the fusion reactor first wall. Bare and hydrogenated amorphous carbon surfaces with temperatures in the range of 300–800 K are bombarded by a distribution of neutral hydrogen molecules representing well-defined center-of-mass and rovibrational temperatures. The MD simulations are benchmarked against experiments in which a heated carbon surface is irradiated with hydrogen molecules from a plasma source. Comparisons between simulations and experiment are presented for the rovibrational distributions upon reflection and the rotational and vibrational accommodation coefficients.

Influence of Surface Termination and Electronic Structure on the Photochemical Grafting of Alkenes to Carbon Surfaces

The ultraviolet-initiated photochemical grafting of n-alkenes to surfaces of amorphous carbon, diamond, and carbon nanofibers has recently emerged as a way to impart new surface properties to these materials. Recent studies have shown that grafting on these materials is initiated by a novel photoelectron ejection process, evidenced by a strong dependence of reaction efficiency on the electron affinity of the reactant molecules. Yet, the role of different surface functional groups and the resulting changes in valence band density of states and surface work function have not been determined previously. Understanding the influence of surface carbonyl groups is particularly significant because C═O groups increase the surface work function but are also known to catalyze certain electron-transfer processes at carbon surfaces. Here, we use infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) to investigate how changes in surface termination (H-termination vs O-termination) and surface annealing impact the valence electronic structure and work function of the samples, and how these changes influence the grafting of alkenes to the surfaces. Four different n-alkenes bearing terminal groups were used in order to identify the role of the molecular electron affinity, using alkenes terminated with -NHCOCF3, -NHCOO(tert-butyl), -COOMe, and -CH3 groups. By using narrowly spaced interdigitated electrodes of amorphous carbon, we are able to directly detect the UV-induced photoemission. Our results show that O-termination of amorphous carbon surfaces enhances the photochemical grafting yield compared with H-termination of surfaces, contrary to what has been found on diamond surfaces. Surfaces annealed to increase the amount of sp2-hybridized carbon, and therefore being most metallic in character, have the highest reactivity. The changes in reactivity are explained in terms of the changes in valence electronic structure of the samples and the influence of oxygen on the photoelectron emission process that initiates the grafting reaction.

Plasma-surface interactions of hydrogenated carbon

We present a review of our study of interactions of plasma particles (atoms, molecules) with hydrogenated amorphous carbon surfaces typical of plasma-facing divertor tiles and deposited layers in magnetic-fusion reactors. Our computer simulations of these processes are based on classical molecular dynamics simulations, using the best currently available multibody bond-order hydrocarbon potentials. Our research in this field has been focused on the chemical sputtering of carbon surfaces at low impact energies, the most complex of the plasma-surface interactions (PSI). Close collaboration with beam-surface and plasma-surface experiments provides not only theoretical support for the experiments, but also builds suitable benchmarks for our methods and codes, enabling production of theoretical plasma-surface data with increased reliability.

Hydrogen reflection in low-energy collisions with amorphous carbon

Reflection and sticking of hydrogen atoms at amorphous carbon surfaces is studied at impact energies ranging from 0.1 to 50eV using molecular dynamics simulations. We show that the reflection coefficient at the lowest energies is large and is very sensitive to both the many-body potential used in the simulations and the degree of hydrogen enrichment of the surfaces.

Energy and angle spectra of sputtered particles for low-energy deuterium impact of deuterated amorphous carbon

We study the translational, vibrational, and rotational energy spectra of atoms and molecules reflected or sputtered from deuterated amorphous carbon surfaces by impact of low-energy (1–30 eV) deuterium atoms. Both the rovibrational and translational energies of sputtered deuterium molecules are found to be close to 1 eV over the whole impact energy range, with approximate equipartition between rotational and vibrational modes, particularly at the higher impact energies. Sputtered carbon-containing molecules are vibrationally energetic, with rovibrational energies in the range of 1.5–2.5 eV; translational and rotational motions are less energetic, close to 0.5 eV, but hotter, with more energy per degree of freedom. The energy distributions of ejected molecules confirm the partial thermalization of the impact cascade. We also study the angular spectrum of the velocity of the outgoing particles as well as their angular momentum. While the velocity vectors are described well by a cosine distribution, a preferred direction of rotation is found at the lowest energies, with the angular momenta preferentially oriented parallel to the surface.

Confinement effects in monomolecular perfluoropolyether liquid films on amorphous carbon surfaces

The mobility of unstirred molecularly thin perfluoropolyether liquid film Zdol, in contact with amorphous carbon surfaces, is investigated via surface capture (or bonding) kinetics. A transition in the Zdol film mobility from a solid-like to a liquid-like behavior is observed as a function of configuration entropy and temperature. The surface capture kinetics of the molecularly thin Zdol films is then compared to convectively well-stirred bulk Zdol on the same carbon surfaces. In both cases, non-classical surface capture dynamics characterized by a time-dependent rate coefficient is observed.

Photo-induced surface functionalization of carbon surfaces: The role of photoelectron ejection

Carbon-based materials are attractive for a wide range of applications, from biomaterials to fuel cells; however, their effective use often requires controlling the surface chemistry to incorporate recognition moieties or reactive centers. The high stability of carbon also makes it a challenging material to functionalize; recently, the use of ultraviolet light (254nm) to initiate functionalization of carbon surfaces has emerged as a way to obtain carbon/organic interfaces with tailored properties. The authors have investigated the mechanism of covalent grafting of amorphous carbon surfaces with functional organic molecules using the photochemical reaction of terminal alkenes. Measurements comparing the reactivity of different n-alkenes bearing different terminal groups at the terminus opposite the olefin showed pronounced differences in reactivity. They characterized the rate and final coverage of the resulting organic layers using x-ray photoelectron spectroscopy and infrared reflection-absorption spectroscopy. Ultraviolet photoelectron spectroscopy and photocurrent measurements suggested that the reaction involves photoelectron emission from the carbon surface into the liquid phase. Density functional calculations show a strong correlation between the electron affinity of the alkenes and the observed reactivity. The specific terminal group opposite to the olefin was found to play an important role in the stabilization of excess negative charges on the molecule, thus explaining the strong dependence of reactivity on the particular terminal group. These findings suggest that the reaction involves injection of photoelectrons into the alkene acceptor levels, leading to the formation of radical anions in the liquid phase. Finally, the authors demonstrate that the grafting of marginally reactive alkenes can be enhanced by seeding the surface with a small amount of good electron accepting groups. These results provide fundamental new insights into the role of electronic excitations in controlling rates and mechanisms of olefin reactions at surfaces. While demonstrated here for amorphous carbon, these results may also be significant for the UV initiated grafting of olefins on other semiconductors.

Enhancement of Photochemical Grafting of Terminal Alkenes at Surfaces via Molecular Mediators: The Role of Surface-Bound Electron Acceptors

The use of ultraviolet light to functionalize the surface of carbon-based materials with terminal alkenes has emerged as a way to overcome the high chemical stability of these surfaces to create functional interfaces. It was previously shown that surface-bound trifluoroacetic acid protected 10-aminodec-1-ene (TFAAD) can be used to promote the grafting of unreactive alkenes using 254 nm light, but the mechanism by which this enhancement occurs was not understood. Here, we present a detailed study of how surface-bound TFAAD enhances the grafting of organic molecules to the surface of hydrogen-terminated amorphous carbon. Infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy experiments show that pregrafting TFAAD onto carbon surfaces greatly enhances the subsequent grafting of 1-dodecene, dodecane and dodecane-d26 by locally facilitating photoemission of electrons. Using a photopatterned TFAAD “seed” layer, we demonstrate that subsequent grafting of these hydrocarbons is enhanced in regions immediately adjacent to the seed and proceeds almost exclusively parallel to the carbon surface. The roles of liquid-phase radical species, valence band holes, and the photochemical fragmentation of surface species in controlling the overall reaction mechanism are discussed.

Surface energy and hybridization studies of amorphous carbon surfaces

Surface properties of a large number of amorphous carbon (a-C) films have been investigated using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Dense a-C surfaces with variable sp 3/(sp 2 + sp 3) average hybridization were grown using sputtering or pulsed laser deposition (PLD) and were further chemically modified by thermal annealing, ion bombardment or covalent grafting of organic monolayers. The average carbon hybridization, impurity level and mass density, were deduced from XPS and photoelectron energy loss spectroscopy (PEELS). The depth sensitivity of the dispersive (Lifshitz–van der Waals) interaction, estimated at 1–2 nm from the dependence of γ LW on the grafted perflorodecene molecule coverage, is much better than XPS which probes a 3–5 nm depth. The observation of a non-monotonic behavior in the correlation between surface hybridization and electron donor component of surface energy reveals that the average carbon hybridization alone does not describe the entire surface energy physics. The role of π bond clustering in the polar interactions is thus considered and some implications on surface reactivity and mutual interactions with molecular or biomolecular species are discussed.