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.     

Surface roughness and light transmission of biaxially oriented polypropylene films

The effect of thermal history on the transparency of biaxially-oriented polypropylene (BOPP) films was investigated. Compression molded sheets prepared with different cooling rates were biaxially oriented at several temperatures. Correlations were sought between the light transmission measured at 633 nm and the surface roughness as characterized by atomic force microscopy. It was determined that surface roughness on the 100-μm size scale was responsible for a loss in transparency. Surface roughness on the submicron size scale did not affect the transparency. The clearest films were obtained from compression molded sheets with the most homogeneous texture and by orienting at the lowest temperature. POLYM. ENG. SCI., 47:1658–1665, 2007. © 2007 Society of Plastics Engineers     

Molecular dynamics simulations for the growth of diamond-like carbon films from low kinetic energy species

In this study, molecular dynamics simulations using the Brenner potential for hydrocarbons have been used to simulate the formation of diamond-like carbon (DLC) films grown from low-energy hydrocarbon radicals (<2 eV). With these simulations, insight is gained in the processes occurring in this type of deposition. The initial surface is a previously deposited DLC surface; impinging particles include Ar⁺ ions, with an energy of 2 eV, as well as several carbon radicals and molecules, and hydrogen atoms, with an energy of 1 eV. Two different radical flux compositions were examined: in the first condition, only C, C₂, and CH were used as growth species, as well as a large flux of H atoms. In the second condition, the same carbon radicals were considered, as well as the C₂H radical and C₂H₂, C₄H₂, and C₆H₂ molecules, but without the H atom flux. These fluxes are similar to different experimental conditions in an expanding thermal Ar/C₂H₂ plasma (expanding thermal plasma, or ETP), using different influxes of acetylene. Several properties of the resulting films will be presented, focusing mainly on the carbon coordination and the bonding network. The simulations suggest that lowering the acetylene influx results in films having a more extensive bonding network, but with more H incorporated. This leads to more polymeric films having a less diamond-like character, as is expected also from experiments. The aim of this work is twofold. The first objective is to compare the structural composition of the simulated films to the structure of the experimentally deposited films by applying similar conditions. Second, the simulations can give us valuable information about the key mechanisms in the deposition process.     

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.     

The growth of transparent amorphous carbon thin films from coal

Large-area, uniform, transparent amorphous carbon (a-C) thin films were synthesized through simple chemical vapor deposition using coal as the solid carbon source. The atomic force microscopy characterization showed that the synthesized carbon thin film has ∼5 nm thickness with ∼0.55 nm surface roughness. The optical transmittance spectrum showed that the carbon thin film has >96% optical transmittance over the spectral range from 350 nm to 900 nm. The carbon thin films can be transferred to various substrates, which show promise for applications in solar cell, optical and magnetic storage disks, light emitting diodes, photodiodes, and biomedical implants.     

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.     

Stability of carbon nitride films prepared from volatile CN species via atomic transport reactions

Thin CN_x films were deposited by inductively coupled plasma chemical vapour deposition (ICP-CVD) at different substrate temperatures. Instead of the conventional gaseous carbon precursors, a pure carbon mesh was used as carbon source with nitrogen as a carrier/reaction gas. The CN_x films are formed from volatile CN species produced via atomic transport reactions. The deposition rate decreases from 3.2 nm/min at room temperature to almost zero at 150°C. The nitrogen fraction N/(N+C) is at about 0.5, as revealed by various analytical techniques; the composition remains unchanged when varying the substrate temperature. The films are composed mainly of sp² carbon atoms bonded to nitrogen atoms. The CN_x layers are stable upon annealing at temperatures up to 300°C; the surface contamination is removed, while no changes in the nitrogen atomic fraction and in the bonding structure are observed.     

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.     

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 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.     

火柴棒状纳米碳管的制备及其生长机理

以钨酸钠为钨源,氯化钠为诱导剂,通过水热法制备了三氧化钨(WO₃)纳米棒,再以葡萄糖为碳源,经再次水热反应对WO₃表面进行碳包覆,然后在氢气和甲烷混合气氛中反应一段时间获得了具有火柴棒状结构的纳米碳管。采用X射线衍射分析、场发射扫描电子显微镜、透射电子显微镜和X射线能量散射谱等手段对样品的晶型、形貌、微结构和表面化学元素进行了表征与分析。结果表明,样品由纳米碳管和碳化钨(WC)构成。其中,纳米碳管为火柴棒状,长度0.5～1.0 μm,直径100 nm左右;WC颗粒位于纳米碳管内部,其大小决定了火柴棒状纳米碳管的内径。这充分说明WC在碳管的生长过程中充当催化剂的作用。     

Surface and interface morphology and structure of amorphous carbon thin and multilayer films

We review the implementation of X-ray reflection (reflectivity and scattering) techniques for the study of amorphous Carbon (a-C, a-C:H, ta-C) thin and multilayer films and in particular in the determination of the film density and surface and interface morphology, which are intrinsically significant for ultra-thin films. We present studies of various a-C and a-C:H films, which include in particular: i) the morphology of a-C/Si interface, ii) the surface morphology and density evolution during sputter growth of a-C, iii) the morphology of the sp²-rich a-C/sp³-rich a-C interfaces in multilayer a-C films, iv) the universal correlation between the film density and the refractive index of a-C and a-C:H films. We also compare and validate the experimental results with relative results from Monte-Carlo simulations within an empirical potential scheme. The computational results shed light on the atomistic mechanisms determining the structure and morphology of the a-C interfaces between individual sp²- and sp³-rich a-C layers and between a-C and Si substrates.     

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 acoustic waves in diamond-like carbon films on LiNbO3

Diamond-like carbon (DLC) films have been reliably deposited on YZ LiNbO_3, and surface acoustic wave (SAW) velocity change has been observed. These relatively thin films increase the SAW velocity sufficiently, and they can be used for SAW devices. The cut-off behavior was observed at approximately 538 MHz in a 2-μm-thick DLC film on the LiNbO₃ substrate.     

Field emission from nano-cluster carbon films

Field emission has been reported to occur at much lower fields in carbon based thin film systems than from any other material systems. The emission has been shown to depend on the various material parameters, but whichever carbon based system is used, it is found that emission occurs at localised sites rather than uniformly over the entire surface. Carbon films with mixed sp³/sp² bonding, like nanocrystalline diamond and nanocluster graphitic films emit at lower fields with a higher emission site density than single-phase films. The sp² cluster size in any carbon film can be altered during deposition, but it is easier to control nanocluster size by post-deposition annealing. Annealing increases the sp² cluster size embedded in a sp³ matrix until the sp³ matrix disappears completely and the film transforms into nanocrystalline graphite. To distinguish the effects of the sp² cluster size from other material parameters, a series of different carbon films were annealed post-deposition and the sp² cluster size was measured using visible Raman. Field emission was then measured at a vacuum of 10⁻⁸ mbar on all films using a parallel plate configuration. It was found that the field emission for all films tested depended upon the clustering of the sp² phase and this effect dominates the effects of the other parameters, such as chemical composition, surface termination, sp³ content or conductivity. The optimum size of the sp² was of the order of 1 nm for all systems tested. We believe that field emission occurs form the localised conducting, predominantly sp² bonded regions, which provideds the large field enhancement required for effective emission.     

Surface modification of amorphous carbon thin films by 1,3-dipolar cycloaddition

Amorphous carbon thin film surfaces were successfully modified by 1,3-dipolar cycloaddition of nitrones, generated by the condensation of 4-(trifluoromethyl)benzaldehyde and N-methylhydroxylamine. Amorphous carbon thin films were deposited by electron cyclotron resonance sputtering and consisted of mainly sp²-hybridized carbon. The modification of amorphous carbon thin film surfaces with organic molecules was confirmed by X-ray photoelectron spectroscopy (XPS), Raman, and atomic force microscopy (AFM). F 1s, N 1s, and C 1s electron spectra revealed the existence of organic molecules on the surface of modified amorphous carbon thin films. The surface coverage increased with reaction temperature, reactant concentration, and reaction time.     

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 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.