Lamp-assisted CVD of carbon micro/nano-structures using metal catalysts and CH2I2 precursor

Carbon micro/nanofibers and nanotubes were deposited via chemical vapor deposition (CVD) using CH₂I₂ precursor and different metal catalysts (Pd, Ni, Fe, Co and Mn) on Si (1 0 0) substrates. A versatile and low-cost IR lamp technique is employed to induce the deposition process. With this method carbon features could be obtained already at temperatures much lower than with common techniques. Palladium metal was deposited by laser-assisted CVD from a liquid solution of the ammine complex and the 3d metals by thermal evaporation. Large-scale periodicity of nano-sized metal catalysts, and subsequently of carbon deposits was obtained by using monolayers of polystyrene microspheres as mask. The carbon structures were analyzed by SEM and micro-Raman spectroscopy.     

Investigation of oxidation resistance of Ni-Ti film used as oxygen diffusion barrier layer

Ni-Ti films prepared at 10 W and 70 W by rf magnetron sputtering are investigated as the oxygen diffusion barrier layer, it is found that crystallinity of Ni-Ti film does not greatly depend on the deposition power. X-ray photoelectron spectroscopy indicates that Ni is still in the form of metallic state from the binding energies of both Ni 2p_3/2 and Ni 2p_1/2 spectra for the sample with 10 W prepared Ni-Ti, however, Ni is oxidized for 70 W prepared Ni-Ti film. Moreover, the (La_0.5Sr_0.5)CoO₃/Pb(Zr_0.40Ti_0.60)O₃/(La_0.5Sr_0.5)CoO₃ capacitor grown on high power prepared Ni-Ti film is leaky, however, the capacitor on low power prepared Ni-Ti film possesses very promising physical properties (i.e. remnant polarization of ∼27 μC/cm² at 5 V and maximum dielectric constant of 940). Leakage current density of the capacitor grown on low power prepared Ni-Ti film is further investigated, it meets ohmic behavior (<1.0 V) and agrees well with the space-charge-limited current theory (>1.0 V).     

Thin graphite overlayers: Graphene and alkali metal intercalation

Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the π states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage 1 compounds C₈K or C₈Rb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 × 2) ordered alkali metal layer adjacent to the uppermost carbon layer.     

Characteristics of Ni films deposited on SiO2/Si(1 0 0) and MgO(0 0 1) by direct current magnetron sputtering system with the oblique target

280 nm-thick Ni films were deposited on SiO₂/Si(1 0 0) and MgO(0 0 1) substrates at 300 K, 513 K and 663 K by a direct current magnetron sputtering system with the oblique target. The films deposited at 300 K mainly have a [1 1 0] crystalline orientation in the film growth direction. The [1 1 0]-orientation weakens and the [1 1 1]- and [1 0 0]-orientations enhance with increasing deposition temperature. The lattice constant of the Ni films is smaller than that of the Ni bulk, except for the film grown on MgO(0 0 1) at 663 K. Furthermore, as the deposition temperature increases, the lattice constant of the films grown on the SiO₂/Si(1 0 0) decreases whereas that of the films grown on the MgO(0 0 1) increases. The films deposited at 300 K and 513 K grow with columnar grains perpendicular to the substrate. For the films deposited at 663 K, however, the columnar grain structure is destroyed, i.e., an about 50 nm-thick layer consisting of granular grains is formed at the interface between the film and the substrate and then large grains grow on the layer. The Ni films deposited at 300 K consist of thin columnar grains and have many voids at the grain boundaries. The grains become thick and the voids decrease with increasing deposition temperature. The resistivity of the film decreases and the saturation magnetization increases with increasing deposition temperature.     

Low energy repetitive miniature plasma focus device as high deposition rate facility for synthesis of DLC thin films

Diamond-like carbon (DLC) films were deposited on Si (1 0 0) substrate using a low energy (219 J) repetitive (1 Hz) miniature plasma focus device. DLC thin film samples were deposited using 10, 20, 50, 100 and 200 focus shots with hydrogen as filling gas at 0.25 mbar. The deposited samples were analyzed by XRD, Raman Spectroscopy, SEM and XPS. XRD results exhibited the diffraction peaks related to SiO₂, carbon and SiC. Raman studies verified the formation amorphous carbon with D and G peaks. Corresponding variation in the line width (FWHM) of the D and G positions along with change in intensity ratio (I_D/I_G) in DLC films was investigated as a function of number of deposition shots. XPS confirmed the formation sp² (graphite like) and sp³ (diamond like) carbon. The cross-sectional SEM images establish the 220 W repetitive miniature plasma focus device as the high deposition rate facility for DLC with average deposition rate of about 250 nm/min.     

A study on magnetic properties and structure of SmCo5 thin films on Ni-W underlayers

A systematic study on Ni-W alloy underlayers has shown that a highly textured (2 1 1)-Ni₄W can be formed after deposition at room temperature. Highly textured (0 0 0 1)-SmCo₅ with a high out-of-plane coercivity (over 10 kOe) and large perpendicular anisotropy can be obtained after deposition on the (2 2 1)-Ni₄W underlayer probably due to a small mismatch between (2 2 1)-Ni₄W and (0 0 0 1)-SmCo₅. Our study indicates that the surface roughness of the underlayers also plays a crucial role, that a smooth surface is favorable for a good crystallinity and high coercivity of SmCo₅. Moreover, we found that a highly textured Ni-(1 1 1) can be obtained on the top of the (2 1 1)-textured Ni₄W. The film structure of SmCo₅/Ni/Ni₄W may be interesting as the hard/soft double-layered film for perpendicular magnetic recording or for other applications after a further development.     

Ultrathin nickel oxide films grown on Ag(0 0 1): a study by XPS, LEIS and LEED intensity analysis

The structure of a nickel oxide film 2 ML thick has been investigated by LEED intensity analysis. The NiO film was prepared by evaporating Ni in presence of O₂ at a pressure in the 10⁻⁶ mbar range. The growth of the oxide film was followed by XPS, LEIS and LEED. In the early stages of deposition, the film shows a (2 × 1) superstructure in LEED. After deposition of 2 ML of NiO, a sharp (1 × 1) LEED pattern is observed. The intensity versus electron energy curves of the LEED spots were measured for this NiO(1 × 1) film and analysed by means of the tensor LEED method. A good level of agreement of the experimental LEED intensities with those calculated for a pseudomorphic NiO(0 0 1) film was obtained. We found that oxygen atoms at the oxide-substrate interface are on-top silver atoms. The interlayer distance in the oxide does not differ significantly from that in bulk NiO(0 0 1), within the accuracy of the analysis. An outward displacement (0.05 ± 0.05 Å) of oxygen atoms with respect to nickel atoms was found at the oxide film surface. The interlayer distance at the silver-nickel oxide interface is 2.43 ± 0.05 Å.     

X-ray multiple diffraction in the characterization of TiNO and TiO2 thin films grown on Si(0 0 1)

TiO₂ and TiN_xO_y thin films grown by low pressure metal-organic chemical vapor deposition (LP-MOCVD) on top of Si(0 0 1) substrate were characterized by X-ray multiple diffraction. X-ray reflectivity analysis of TiO₂[1 1 0] and TiNO[1 0 0] polycrystalline layers allowed to determine the growth rate (−80 Å/min) of TiO₂ and (−40 Å/min) of TiNO films. X-ray multiple diffraction through the Renninger scans, i.e., ?-scans for (0 0 2)Si substrate primary reflection is used as a non-conventional method to obtain the substrate lattice parameter distortion due to the thin film conventional deposition, from where the information on film strain type is obtained.     

Temperature and substrate dependence of structure and growth mechanism of carbon nanofiber

The carbon nanofibers were grown on Ni/Si and Ni/Ti/Si substrates in 1 atm CH₄ atmosphere at 640 °C and 700 °C by thermal chemical vapor deposition method. The carbon nanofibers were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectrometry for morphology, microstructure, and crystallinity. The electron emission property of carbon nanofibers was also investigated by current-voltage (I-V) measurement. The results showed that the solid amorphous carbon nanofibers could be grown on Ni/Si substrate at 640 °C through tip growth mechanism, the carbon nanotubes could be grown on Ni/Si substrate at 700 °C through tip growth mechanism, and the carbon nanotubes could be grown on Ni/Ti/Si substrate at 700 °C through root growth mechanism.     

Study on effects of substrate temperature on growth and structure of alignment carbon nanotubes in plasma-enhanced hot filament chemical vapor deposition system

Alignment carbon nanotubes (ACNTs) were synthesized on silicon substrate coated with Ni catalyst film and Ta buffer layer by plasma-enhanced hot filament chemical vapor deposition using CH₄, NH₃, and H₂ as the reaction gas, and they were investigated by scanning electron microscopy and transmission electron microscopy. It is found that the diameter of the bamboo-structured ACNTs is increased from 62 to 177 nm when the substrate temperature was changed from 626 to 756 °C. Their growth rate is enhanced by the substrate temperature in a range of 626-683 °C and it is reversely reduced with the substrate temperature after the substrate temperature is over 683 °C. Beginning with wetting phenomenon, the effects of the substrate temperature on the structure and growth rate of the ACNTs are analyzed.     

Kinetic aspect of CO2 reforming of CH4 on Ni(1 1 1): A density functional theory calculation

Reaction pathways of CO₂ reforming of CH₄ on Ni(1 1 1) were investigated by using density functional theory calculation. The computed kinetic parameters agree with the available experimental data, and a new and simplified mechanism was proposed on the basis of computed energy barriers. The first step is CO₂ dissociation into surface CO and O (CO₂ → CO + O) and CH₄ sequentially dissociation into surface CH and H (CH₄ → CH₃ → CH₂ → CH). The second step is CH oxygenation into CHO (CH + O → CHO), which is more favored than its dissociation into C and hydrogen (CH → C + H). The third step is the dissociation of CHO into surface CO and H (CHO → CO + H). Finally, H₂ and CO desorb from Ni(1 1 1) and form free H₂ and CO. The rate-determining step is the CH₄ dissociative adsorption, and the key intermediate is surface adsorbed CHO. Parameters, which might modify the proposed mechanism, have been analyzed. In addition, the formation, deposition and elimination of surface carbon have been discussed accordingly.     

Oxygen reduction activity of N-doped carbon-based films prepared by pulsed laser deposition

Carbon-based films with nitrogen species on their surface were prepared on a glassy carbon (GC) substrate for application as a non-platinum cathode catalyst for polymer electrolyte fuel cells. Cobalt and carbon were deposited in the presence of N₂ gas using a pulsed laser deposition method and then the metal Co was removed by HCl-washing treatment. Oxygen reduction reaction (ORR) activity was electrochemically determined using a rotating disk electrode system in which the film samples on the GC substrate were replaceable. The ORR activity increased with the temperature of the GC substrate during deposition. A carbon-based film prepared at 600 °C in the presence of N₂ at 66.7 Pa showed the highest ORR activity among the tested samples (0.66 V vs. NHE). This film was composed of amorphous carbons doped with pyridine type nitrogen atoms on its surface.     

Self assembly of SiO2-encapsulated carbon microsphere composites

SiO₂ was firstly coated onto the surface of carbon microspheres (CMSs) using tetraethyl orthosilicate (TEOS) as precursor by Stöber method. Then SiO₂-encapsulated CMS (CMS@SiO₂) composites were self-assembled by vertical deposition, in which the effects of deposition temperature and suspension concentration on the quality of self-assembling film were investigated. Morphologies and structures of the samples were characterized by field emission scanning electron microscopy, Fourier transformation infrared spectrometry, X-ray diffraction and thermogravimetry. The results show that uniform CMS@SiO₂ composites with good mono-dispersion were prepared by St?ber method with 0.5 g of CMSs, 2 mL of TEOS, 30 mL of ammonia and 12 h of reaction time, the CMSs-based films with ordered and denser structure were prepared by vertical deposition using CMS@SiO₂ composites as monodipersion spheres under suspension concentration of 1 wt% and deposition temperature of 50 °C. The ultraviolet-visible absorption measurement shows that the absorbance of CMS@SiO₂ composite films grew steadily with increasing suspension concentration.     

Growth and oxidation of a Ni3Al alloy on Ni(1 0 0)

The growth and oxidation of a thin film of Ni₃Al grown on Ni(1 0 0) were studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). At 300 K, a 12 Å thick layer of aluminium was deposited on a Ni(1 0 0) surface and subsequently annealed to 1150 K resulting in a thin film of Ni₃Al which grows with the (1 0 0) plane parallel to the (1 0 0) surface of the substrate. Oxidation at 300 K of Ni₃Al/Ni(1 0 0) until saturation leads to the growth of an aluminium oxide layer consisting of different alumina phases. By annealing up to 1000 K, a well ordered film of the Al₂O₃ film is formed which exhibits in the EEL spectra Fuchs-Kliewer phonons at 420, 640 and 880 cm⁻¹. The LEED pattern of the oxide shows a twelvefold ring structure. This LEED pattern is explained by two domains with hexagonal structure which are rotated by 90° with respect to each other. The lattice constant of the hexagonal structure amounts to ∼2.87 Å. The EELS data and the LEED pattern suggest that the γ^′-Al₂O₃ phase is formed which grows with the (1 1 1) plane parallel to the Ni(1 0 0) surface.     

Study of structural and electronic environments of hydrogenated amorphous silicon carbonitride (a-SiCN:H) films deposited by hot wire chemical vapor deposition

Hydrogenated amorphous silicon carbon nitride (a-SiCN:H) thin films were deposited by hot wire chemical vapor deposition (HWCVD) using SiH₄, CH₄, NH₃ and H₂ as precursors. The effects of the H₂ dilution on structural and chemical bonding of a-SiCN:H has been investigated by Raman and X-ray photoelectron spectroscopy (XPS). Increasing the H₂ flow rate in the precursor gas more carbon is introduced into the a-SiCN:H network resulting in decrease of silicon content in the film from 41 at.% to 28.8 at.% and sp² carbon cluster increases when H₂ flow rate is increased from 0 to 20 sccm.     

Plasma characterization and room temperature growth of carbon nanotubes and nano-onions by excimer laser ablation

The deposition of carbon nanotubes and carbon nano-onions at room temperature using excimer laser radiation to ablate mixed graphite-metal targets is described. Our deposition conditions are in contrast to other investigations on the pulsed laser deposition of carbon nanotubes that have employed high temperatures and high pressures. We find that the formation of these carbon nanostructures is dependent on the ambient gas employed during ablation. In the presence of O₂ gas, carbon nanotubes and nano-onions are produced, while inert atmospheres such as Ar yield amorphous carbon. High-resolution, in situ, time-resolved emission spectroscopy has been used to track the evolution of species (C₂, C₃, Ni/Co) in the ablation plume. Spectral fits on low and high-resolution spectra reveal that the vibrational-rotational temperatures for C₂ produced in O₂ remain at ∼5000 K for nearly 20 μs but drop rapidly in Ar. Details of the formation of carbon nanotubes and nano-onions, and in situ time-resolved optical emission spectroscopy are described.     

Structural evolution and epitaxial stabilisation of pulsed laser deposited Sm0.55Nd0.45NiO3 solid solution nanostructured films on undoped Si (1 0 0) and NdGaO3 substrates

We report on the effects of substrate, ambient oxygen pressure and deposition time on the crystal structure, and morphology of Sm_0.55Nd_0.45NiO₃ solid solution nanostructured films synthesized by pulsed-laser deposition. In each film the structure was found to be consistent with a perovskite structure with preferential planes growth and reveals a strong orientation along the orthorhombic (2 1 0) plane of the perovskite subcell for the film deposited on NdGaO₃ where highly crystalline films were obtained within 15 min deposition time with a low surface roughness of 8.79 nm. Similar structure is observed on Si (1 0 0) substrate only at O₂ pressure of 0.4 mbar. The surface morphology of the different samples shows a net dense film structure with several droplets population. The nano-scaled droplets are in general spherical in shape; a detailed analysis indicates that the laser ablation of this nickelate family is governed to a certain extent by a heat transfer phenomenon.     

Fabrication of multiwalled carbon nanotubes in the channels of iron loaded three dimensional mesoporous material by catalytic chemical vapour deposition technique

The growth of multiwalled carbon nanotubes (MWNTs) was successfully achieved in the channels of three dimensional (3D) iron loaded mesoporous matrices (KIT-6) by employing catalytic chemical vapour deposition (CCVD) technique. The synthesised MWNTs, which were characterised by SEM, TEM and Raman spectroscopy, consist of thick graphene layers of about 10 nm composed of 29 graphene sheets with inner and outer diameter of ∼17 nm and ∼37 nm, respectively. The Raman spectrum showed the formation of well-graphitised MWNTs with significantly higher I_G/I_D ratio of 1.47 compared to commercial MWNTs. Comparatively, 2 wt% Fe loaded KIT-6 material produced a better yield of 91%, which is also highest compared with the report of MWNTs synthesis using mesoporous materials reported so far.     

Effects of rapid thermal annealing on structural, magnetic and optical properties of Ni-doped ZnO thin films

XPS depth profiles were used to investigate the effects of rapid thermal annealing under varying conditions on the structural, magnetic and optical properties of Ni-doped ZnO thin films. Oxidization of metallic Ni from its metallic state to two-valence oxidation state occurred in the film annealed in air at 600 °C, while reduction of Ni²⁺ from its two-valence oxidation state to metallic state occurred in the film annealed in Ar at 600 and 800 °C. In addition, there appeared to be significant diffusion of Ni from the bottom to the top surface of the film during annealing in Ar at 800 °C. Both as-deposited and annealed thin films displayed obvious room temperature ferromagnetism (RTFM) which was from metallic Ni, Ni²⁺ or both with two distinct mechanisms. Furthermore, a significant improvement in saturation magnetization (M_s) in the films was observed after annealing in air (M_s = 0.036 μ_B/Ni) or Ar (M_s = 0.033 μ_B/Ni) at 600 °C compared to that in as-deposited film (M_s = 0.017 μ_B/Ni). An even higher M_s value was observed in the film annealed in Ar at 800 °C (M_s = 0.055 μ_B/Ni) compared to that at 600 °C mainly due to the diffusion of Ni. The ultraviolet emission of the Ni-doped ZnO thin film was restored during annealing in Ar at 800 °C, which was also attributed to the diffusion of Ni.