Surface morphology and evolution of amorphous carbon thin films

The surface morphology of disordered carbon films grown by nanosecond pulsed laser ablation of graphite is reviewed. It is shown that the presence of a background gas can have a profound effect on the plume of material ejected during ablation. At low pressures smooth films are produced but at higher pressures rough films with an evolution from a nodular morphology to a large area cluster-assembled morphology occurs. The surface morphology changes with increasing background pressure as a result of collisions, which reduce the kinetic energy of the ejected material and allow for cluster formation within the plume. It is shown that the energy of some of the carbon ablated species in vacuum can exceed 100 eV. The nature of the species present in the plume is discussed in terms of electron–ion recombination and impact ionisation/excitation. The cluster-assembled films are shown to be useful as a scaffold for supporting metal nanoparticles to produce substrates for surface enhanced Raman spectroscopy.     

Electrodeposition mechanism of hydrogen-free diamond-like carbon films from organic electrolytes

A brief introduction on the development of electrodeposition of diamond-like carbon (DLC) films was given, and our experiments were done, emphasizing on how to deposit hydrogen-free DLC films. Methanol, acetonitrile and N,N-dimethyl formamide (DMF) were chosen as electrolytes, while Si and conductive glass were used as substrates. The sample deposited on Si through methanol was the only one in this comparative research that produced hydrogen-free DLC film as it was indicated by the FTIR spectroscopy. Two explanations, based on reaction mechanism, were proposed to explain this fact. It was believed that the reaction rate and the effect of hydroxyl groups in the molecules of the electrolytes played important roles in the deposition of hydrogen-free DLC films.     

Surface roughness of thin silver films pulse-plated using silver cyanide-thiocyanate electrolyte

We investigated the effect of the pulse current density, current-on time and current-off time on the surface roughness of a silver thin film that was pulse-plated using a silver cyanide-thiocyanate electrolyte. The interface width, which is defined by the root mean square of the fluctuations in the height of the surface, is found to decrease rapidly with the current-off time and to increase with current-on time. However, it should be noted that when the value of current-off time is sufficiently large, the interface width decreases with pulse current density. These experimental results indicate that by appropriate selection of the current-on time and current-off time, we can fabricate silver electrodeposits with minimal surface roughness. We have discussed the influence of these three parameters on the interface width from the viewpoint of the cathode potential and adsorption.     

Surface roughness of AlN films deposited on negatively biased silicon and diamond substrates

Our previous studies on AlN microstructures have shown that smooth amorphous films (a-AlN) can be grown on negatively biased Si substrates by the versatile physical vapour deposition technique under reactive magnetron sputtering. These a-AlN films are produced by energetic Ar ion bombardment under negative bias whereas those grown without bias were columnar crystallized ones (c-AlN). Here, we show first that depositing an a-AlN layer on c-AlN/Si structures by switching a suitable bias to the Si substrate can efficiently reduce their surface roughness. We then extend this smoothening method to a c-AlN/Poly-crystallized diamond (PCD) structure to reduce its high surface roughness that hampers using such structures in SAW device design. In fact, the piezoelectric c-AlN surfaces grown on rough diamond surfaces are equally rough. Effectively, the a-AlN layer deposited on the c-AlN/PCD structure brings down the latter's RMS surface roughness to one tenth of its initial RMS roughness, as confirmed here by TEM and AFM observations. The insulating property of the diamond as biased substrate doesn't impede the growth of this a-AlN layer. This smoothening method is without process interruption, where simply a negative bias is switched on to the diamond substrate once the desired piezoelectric c-AlN film thickness as monitored here by in-situ reflectometry, is attained. This as-grown smoothening method can be therefore easily and rapidly implemented and can thus replace time-consuming and costly PCD ionic and/or mechanical polishing. Hopefully, the method can be advantageously applied to c-AlN/nano-crystallized diamond structures (NCD) where the NCD films are not prepared under rigorous conditions meant to minimize their surface roughness.     

Bubble growth mechanism in carbon foams

The present work is a numerical study to predict the growth mechanism of a non-spherical bubble assisted for a carbon foam fabrication process. An approach for two dimensional non-spherical mass-diffusion controlled bubble growth in an isothermal Newtonian liquid of infinite extent is considered. Using the two dimensional unsteady form of the equations governing the conservation of mass and momentum, bubble growth is solved as a function of time using a fixed-grid sharp interface finite volume method. A comparative study is performed by considering previous cases of study and shows good agreement, which reflects the validity of the present model. A parametric study highlighting the effects of the non-spherical growth of the bubble is performed in order to emphasize how controlled bubble growth can be achieved. In each case a change in a particular parameter resulted in a distinct change of the bubble shape.     

Solid-liquid-solid growth mechanism of single-wall carbon nanotubes

Reasons are presented which suggest that the liquefaction of the catalytic particles is a decisive condition for formation of single wall carbon nanotubes (SWNTs) by physical synthesis techniques. It is argued that the SWNT growth mechanism is a kind of solid-liquid-solid graphitization of amorphous carbon or other imperfect carbon forms catalyzed by molten supersaturated carbon-metal nanoparticles. The assumption of low temperature melting of these nanoparticles in contact with amorphous carbon followed by its precipitation in the form of SWNTs allows to explain qualitatively the experimentally observed SWNT growth rates and temperature dependence of the SWNT yield. Guidelines for increasing SWNT yield are proposed.     

Surface modification of nanocrystalline diamond/amorphous carbon composite films

The surfaces of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films deposited from a 17% CH₄/N₂ mixture have been subjected to a variety of plasma and chemical treatments, namely H₂ and O₂ microwave plasmas, a CHF₃ 13.56 MHz plasma, and a chemical treatment with aqua regia (HCl:HNO₃ 3:1). The resulting surfaces have been studied with respect to their chemical nature by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS), concerning their morphology with atomic force microscopy, and by contact angle measurements to study their hydrophobicity and their stability. As-grown surfaces are hydrogen terminated, but the number of C–H bonds can slightly be increased by a H₂ microwave plasma, while treatment with aqua regia considerably lowers the number of C–H bonds at the surface. O₂ and CHF₃ plasmas, on the other hand, lead to a replacement of the terminating C–H bonds by C–O or C–OH and C–F_x groups, respectively. Finally, by contact angle measurements over a period of 150 days it could be shown that the H-terminated surface is very stable whereas the contact angle of the O-treated surface changed considerably with time, probably due to the adsorption of contaminants.     

Transfer-free growth of graphene on SiO2 insulator substrate from sputtered carbon and nickel films

Here we demonstrate the growth of transfer-free graphene on SiO₂ insulator substrates from sputtered carbon and metal layers with rapid thermal processing in the same evacuation. It was found that graphene always grows atop the stack and in close contact with the Ni. Raman spectra typical of high quality exfoliated monolayer graphene were obtained for samples under optimised conditions with monolayer surface coverage of up to 40% and overall graphene surface coverage of over 90%. Transfer-free graphene is produced on SiO₂ substrates with the removal of Ni in acid when Ni thickness is below 100 nm, which effectively eliminates the need to transfer graphene from metal to insulator substrates and paves the way to mass production of graphene directly on insulator substrates. The characteristics of Raman spectrum depend on the size of Ni grains, which in turn depend on the thickness of Ni, layer deposition sequence of the stack and RTP temperature. The mechanism of the transfer-free growth process was studied by AFM in combination with Raman. A model is proposed to depict the graphene growth process. Results also suggest a monolayer self-limiting growth for graphene on individual Ni grains.     

Surface chemistry and adsorption mechanism of cadmium ion on activated carbon derived from Garcinia mangostana shell

A detailed surface characterizations and adsorption mechanism of Cd²⁺ on chemical activated carbon (CAC) prepared from Garnicia mangostana shell were investigated. The activation is accomplished in self-generating atmosphere using phosphoric acid as activating agent. The characterizations performed are elemental analysis, functional group identification, N₂ adsorption isotherm and surface charges. Adsorption mechanism of metal ion was tested using Cd²⁺ as model ion. CAC achieved BET surface area of 1,498 m²/g with a mixture of micro and mesopores. The point of zero charge is observed to be at pH 2.8 and the optimum pH for Cd²⁺ adsorption on CAC is 12. The adsorption isotherm followed the Freundlich model, and the adsorption kinetics was explained by pseudo-second order kinetic model. From thermodynamic studies, the adsorption was found to be physical adsorption. X-ray photoelectron spectroscopy (XPS) confirmed the adsorption of Cd²⁺ onto CAC as +2 oxidation state.     

Surface energy of the plasma treated Si incorporated diamond-like carbon films

Surface energy and surface chemical bonds of the plasma treated Si incorporated diamond-like carbon films (Si-DLC) were investigated. The Si-DLC films were prepared by r.f. plasma assisted chemical vapor deposition using benzene and diluted silane (SiH₄/H₂ = 10:90) as the precursor gases. The Si-DLC films were subjected to plasma treatment using various gases like N₂, O₂, H₂ and CF₄. The plasma treated Si-DLC films showed a wide range of water contact angles from 13.4° to 92.1°. The surface energies of the plasma treated Si-DLC films revealed a high polar component for O₂ plasma treated Si-DLC films and a low polar component for CF₄ plasma treated Si-DLC films. The CF₄ plasma treated Si-DLC films indicated the minimum surface energy. X-ray photoelectron spectroscopy (XPS) revealed that the polarizability of the bonds present on the surface explains the hydrophilicity and hydrophobicity of the plasma treated Si-DLC films. We also suggest that the O₂ plasma treated surface can provide an excellent hemocompatible surface from the estimated interfacial energy between the plasma treated Si-DLC surface and human blood.     

Surface modification of polymeric nanocomposite thin films using supercritical carbon dioxide

We report on an efficient and environmentally friendly means to modify surface properties of polymer films supported for nanoparticles. Ultrathin polystyrene (PS) films (<300 Å), in which inorganic nanoparticles were embedded, were exposed to supercritical carbon dioxide (scCO₂). The swollen structure was then preserved by quickly evaporating CO₂. X-ray reflectivity (XR) results showed that this procedure produced polymeric nanocomposite films with a low-density region of about 150Å at the polymer/air interface. The formation of the low-density layer was independent of the nature of the particles, indicating that the surface modification through exposure to scCO₂ may be a universal phenomenon regardless of a choice of nanoparticles.     

Surface roughness analysis of electrodeposited Cu

Cu films were electrodeposited with mass transport controlled using a rotating disc electrode (RDE), and imaged with an atomic force microscope (AFM). The length-dependent roughness w(l,t) of these films follows a power law of the form w∝lHtβloc for small length-scales l, with the local roughness exponent, β_loc, varying from 0 to 0.5 depending on the experimental conditions. It was found that contrary to previous work β_loc is not simply a function of the ratio of the current j to its diffusion-limited value j_L. Focused ion beam (FIB) imaging was used as a new method of characterizing the film roughness. FIB images confirmed the existence of small β_loc values for films for which the AFM data could have been unreliable. FIB is a particularly powerful method for characterizing high roughness films.     

Kinetics of deposition of pyrolytic carbon films from methane and carbon suboxide

Using an isothermal flow reactor of annular configuration, the rates of carbon deposition from CH₄ and C₃O₂ have been measured over the respective temperature ranges, 1166°–1372°K and 900°–1025°K. Under the experimental conditions employed, deposition from both of these compounds is found to be controlled by their rates of unimolecular decomposition in the gas phase. Carbon deposition from C₃O₂ is inhibited by CO, in accordance with the postulated mechanism for C₃O₂ decomposition.     

Preparation, characterization and growth mechanism of platelet carbon nanofibers

The synthesis of platelet carbon nanofibers (PCNFs) on a silicon substrate using chemical vapor deposition method is reported. Scanning electron microscope, high-resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the nanofibers. It is found that these platelet nanofibers are of the order of 10 μm long, and most have a nearly rectangular transverse section with several hundreds nm wide and several tens of nm thick. Structure analysis reveals that the carbon layers of platelet nanofibers are parallel to each other, and have a uniform (0 0 2) orientation that is perpendicular to the fiber axis. Many faults and nanodomain have been found in the nanofibers. It is suggested that the PCNF grow in tip growth mechanism by the precipitation of carbon from the side facet of catalyst flakes.     

Effects of carbon film roughness on growth of carbon nanotip arrays by plasma-enhanced hot filament chemical vapor deposition

Carbon nanotip arrays were grown from carbon films deposited on silicon substrates by plasma-enhanced hot filament chemical vapor deposition. The carbon films were characterized by the atomic force microscopy and micro-Raman spectrometry and the carbon nanotip arrays were investigated by micro-Raman spectrometry, scanning electron microscopy and X-ray photon spectroscopy. The results indicate that both carbon films and carbon nanotips are made of polyaromatic carbon with turbostratic structure. Carbon films possess various roughnesses, which greatly influence the nanotip growth rate. The theory related to plasma and sputtering effect is used to discuss the results.     

Controlled growth of carbon nanotube films for high-current field emission

Carbon nanotubes (CNTs) offer great potential for numerous cold-cathode field emission applications. A less studied need is for high-current cathodes. While work to date has focused on the use of tangled webs of single-wall CNTs, much understanding about field emission has occurred from studies using multi-wall CNTs with controlled geometries. However, the crystalline nature of these multi-wall CNTs typically is far inferior to that of single-wall CNTs. We use high-resolution transmission electron microscopy to demonstrate that growth at temperatures ≤ 630 °C via thermal chemical vapor deposition can produce highly crystalline multi-wall CNTs, with structures consisting entirely of concentric graphene cylinders. Conversely, growth at temperatures ≥ 650 °C results in crystalline CNTs embedded in a nanocrystalline graphite, or glassy carbon, sheath. This sheath material is likely a poor electrical conductor, due to phonon scattering, and will have deleterious effects on field emission. Field emission measurements taken from such films are consistent with the best field-emitting multi-wall CNT films in the literature, in terms of total current for a given applied field, but without the benefit of the preferred perpendicular orientation. These results are promising toward the development of reliable high-current field emission cathodes.     

Surface characteristics of carbon fibres from PAN

Surface characteristics of the three types of commercial carbon fibres from PAN, in both treated and untreated forms, have been investigated along with a series of high modulus (HM) fibres oxidised in HNO₃. N₂ adsorption isotherms have been used to elucidate physical characteristics and O₂ chemisorption measurements to determine the area on which surface complexes can form, the so called “active surface area”. Comparative plots show that there is no microporosity developed during surface treatment of fibres, but high temperature degassing does bring about a degree of microporosity, as suggested by Mimeault and McKee[5]. The external area of HM fibres is increased by HNO₃ oxidation but there is little difference between the areas of untreated fibres and those surface treated by the supplier. Both types of treatment, however, increase the active surface areas by factors of 3–30 times, and also increase the fraction of the external surface which is in the form of edge, or active, sites.     

Orientationally ordered and patterned discotic films and carbon films from liquid crystal precursors

This article demonstrates techniques for fabricating novel organic and carbon films, in which liquid crystal surface anchoring and flow are exploited to precisely control molecular structure (in organic films) or crystal structure (in carbon films). Surface anchoring states were first measured for AR mesophase on spin-coatable organic resins, including commercial polyimide and photoresist. These results were used to develop a lithographic technique for ordering AR surfaces in preprogrammed orientational micropatterns. AR was also processed into radial or star symmetry films by forced spreading combined with edge-on anchoring templates. Additional thin films were prepared from alternative liquid crystalline precursors composed of sulfonated polyaromatic dyes. These disk-like planar molecules undergo massive π-stacking in aqueous solution to form rod-like aggregates. At high concentrations or on surfaces, these rods or molecular columns align by repulsive interactions (lyotropic behavior), giving raise to a transverse alignment of the stacked polyaromatic disks. Here the lyotropic dye indanthrone disulfonate is used to make fully dense ordered carbon films by spin coating or Meyer-bar coating thin films on quartz followed by direct carbonization (without oxidative stabilization). These films exhibit surfaces rich in graphene edge-sites and are either anisotropic unidirectional (by bar coating) or multi-domain with long-range isotropy (by spin coating).     

A growth mark method for studying growth mechanism of carbon nanotube arrays

A novel and simple growth mark method was developed to make marks during the growth process of carbon nanotube arrays. These marks can be read out under scanning electron microscope or optical microscope. Based on this method, the growth rates at different temperatures and under different acetylene partial pressures were measured, from which the activation energy and the order of reaction were determined. Based on our experimental results, the growth of carbon nanotube arrays in our experimental condition could not be diffusion-limited. The measured activation energy could possibly be attributed to the heterogeneous decomposition of acetylene over the catalyst particle. Furthermore the marked array with special segmental structure may be found some applications in the future.