Growth and microstructures of carbon nanotube films prepared by microwave plasma enhanced chemical vapor deposition process

Well-aligned carbon nanotubes (CNTs) were grown on iron coated silicon substrates by microwave plasma enhanced chemical vapor deposition. Effect of plasma composition on the growth and microstructures of CNTs were investigated by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and optical emission spectroscopy. Morphology and microstructure of nanotubes were found to be strongly dependent on the plasma composition. Aligned bamboo-shaped nanotubes consisting of regular cone shaped compartments were observed for C₂H₂/NH₃/N₂ and C₂H₂/NH₃/H₂ gas mixtures. Randomly oriented or no nanotubes growth were observed in C₂H₂/H₂ and C₂H₂/N₂ gas mixtures respectively. CNTs grown in nitrogen rich plasma had more frequent short compartments while compartment length increased with decreasing nitrogen concentration in the plasma. Raman spectroscopy of CNTs samples revealed that CNTs prepared in nitrogen rich plasma had higher degree of disorder than those in low nitrogen or nitrogen free plasma. In-situ optical emission spectroscopy investigations showed that CN and H radicals play very important role in both the growth and microstructure of CNTs. Microstructure of CNTs has been correlated as a function of CN radical concentration in the plasma. It is suggested that presence of nitrogen in the plasma enhances the bulk diffusion of carbon through the iron catalyst particles which causes compartment formation. Based on our experimental observations, growth model of nanotubes under different plasma composition has been suggested using base growth mechanism.     

Optical characterization of BCN films deposited at various N2/Ar gas flow ratios by RF magnetron sputtering

We present the deposition and optical characterization of amorphous thin films of boron carbonitride (BCN). The BCN thin films were deposited in a radio frequency magnetron sputtering system using a B₄C target. Films of different compositions were deposited by varying the ratio of argon and nitrogen gas in the sputtering ambient. X-ray photoelectron spectroscopy was used to perform surface characterization of the deposited films and a change in composition with nitrogen flow ratio was observed. The effect of gas flow ratios on the optical properties of the films was also investigated. It was found that the transmittance of the films increases with nitrogen incorporation. The optical band gap of the films ranged from 2.0 eV to 3.1 eV and increased with N₂/Ar gas flow ratio except at the highest ratio.     

Gas sensors based on doped-CNT/SnO2 composites for NO2 detection at room temperature

For the first time nitrogen or boron doped carbon nanotubes were added into a SnO₂ matrix to develop a new hybrid CNTs/SnO₂ gas sensors. The hybrid sensor is utilised to detect low ppb concentrations of NO₂ in air, by measuring resistance changes of thin CNTs/SnO₂ films. The tests are performed at room temperature. For comparison, pure SnO₂ and N or B-substituted CNT sensors are also examined. Comparative gas sensing results reveal that the CNTs/SnO₂ hybrid sensors exhibit much higher response towards NO₂, at least by a factor of 10, and good baseline recovery properties at room temperature than the blank SnO₂ and the N or B-substituted CNT sensors. This finding shows that doping SnO₂ with low quantity of CNTs doped with heteroatoms can dramatically improve sensitivity.     

Role of carbon atoms in plasma-enhanced chemical vapor deposition for carbon nanotubes synthesis

The role of carbon atoms in a dc plasma-enhanced chemical vapor deposition for carbon nanotubes (CNTs) synthesis was investigated. It was observed that at 1.33 kPa pressure of CH₄ gas in plasma, a high value of the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum corresponds to the maximum value of the excited C number density in the vicinity of the Si substrate. It was found that a CH₄ gas pressure higher than 1.33 kPa leads to an increase of the relative density of the C₂, C₃ molecules and the clusters, and to a decrease of the C excited atom number density in plasma. The presence of a high amount of sp²-graphite in the composition of CNTs observed in Raman spectrum was also confirmed by the measurement of the IR-active G peak at 1584 cm^- 1 in the transmission spectrum.     

Fabrication of diamond-like carbon film on rubber by T-shape filtered-arc-deposition under the influence of various ambient gases

Diamond-like carbon (DLC) films were prepared on rubber substrates using T-shape filtered-arc-deposition (T-FAD), which effectively removes the macrodroplets emitted from the graphite cathode spot from the processing plasma. In the present study, the influence of ambient gas (no gas, Ar, H₂, C₂H₂, C₂H₄, CH₄) was investigated. The DLC films adhered well to the rubber substrate. When the substrate was stretched, the small DLC islands were separated and clefs were opened. The deposition rate on rubber was approximately twice higher than that on a Si substrate. When hydrocarbon gas was introduced as an ambient gas, the deposition rate became higher than that for no gas and H₂ gas. In the cases of C₂H₄ and CH₄ gases, the DLC film was considered to contain a considerable amount of hydrogen. When C₂H₂ gas was used, the highest deposition rate with less surface roughness was achieved.     

Synthesis of nano-carbon (nanotubes,nanofibres, graphene) materials

In the present study, we report the synthesis of carbon nanotubes (CNTs) using a new natural precursor: castor oil. The CNTs were synthesized by spray pyrolysis of castor oil-ferrocene solution at 850°C under an Ar atmosphere. We also report the synthesis of carbon nitrogen (C-N) nanotubes using castor oil-ferrocene-ammonia precursor. The as-grown CNTs and C-N nanotubes were characterized through scanning and transmission electron microscopic techniques. Graphitic nanofibres (GNFs) were synthesized by thermal decomposition of acetylene (C₂H₂) gas using Ni catalyst at 600°C. As-grown GNFs reveal both planar and helical morphology. We have investigated the structural and electrical properties of multi-walled CNTs (MWNTs)-polymer (polyacrylamide (PAM)) composites. The MWNTs-PAM composites were prepared using as purified, with ball milling and functionalized MWNTs by solution cast technique and characterized through SEM. A comparative study has been made on the electrical property of these MWNTs-PAM composites with different MWNTs loadings. It is shown that the ball milling and functionalization of MWNTs improves the dispersion of MWNTs into the polymer matrix. Enhanced electrical conductivity was observed for the MWNTs-PAM composites. Graphene samples were prepared by thermal exfoliation of graphite oxide. XRD analysis confirms the formation of graphene.     

Influence of CO2-laser mixture composition on the distribution of energy in nitrogen spectrum

The DC glow discharge has been studied for mixtures of helium, nitrogen and carbon dioxide - these mixtures are widely used in CO₂ lasers, which are classified as discharge lasers.Analysis of the bands of the nitrogen molecule (the first and the second positive system - B³Π_g → A³Σ_g, C³Π_u → B³Π_g respectively) in an emission spectrum of the glow discharge was used to deduce an energetic balance and a vibrational temperature.The vibrational temperature of dinitrogen was determined by means of optical emission spectroscopy from the second positive system of the nitrogen molecule for various discharge currents and pressures. The dependence of the vibrational temperature on the nitrogen ratio in the mixture was also investigated.A connection between an occurrence of discharge instabilities (often called striations or ionization waves) and the composition of the gas mixture was observed - the less nitrogen was put in the mixture (in favor of helium), the larger part of positive column was subject to standing ionization waves. Ionization waves observed in the positive column of the glow discharge were investigated using optical emission spectroscopy, which showed sinusoidal profile of the vibrational temperature along the axial direction of the ionization waves.     

Magnetic field enhances the graphitized structure and field emission effect of carbon nanotubes

This study uses a low temperature thermal chemical vapor deposition with an applied external magnetic field to grow carbon nanotubes (CNTs) on Ni/Ag-printed glass substrates. A mixture of C₂H₂ and H₂ gas was used for the growth of the CNTs. A Ni catalyst layer was deposited on the Ag-printed glass substrate by pulse electroplating. Scanning electron micrographs as well as the presence of two sharp peaks at 1320 cm⁻¹ (D band) and 1590 cm⁻¹ (G band) in the Raman spectra indicate that the graphitized structure of CNTs synthesized under a magnetic field has higher quality (i.e., a D-band to G-band intensity ratio of 0.303) than CNTs synthesized without a magnetic field. Transmission electron micrographs show a fine Ni catalyst at the tip of the tube for CNTs synthesized under a magnetic field, exhibiting a CNT “tip-growth” model. The synthesis of CNTs in the presence of a magnetic field also generates better field emission properties and better lighting morphology than without a magnetic field.     

Characterization of SiO2 and TiO2 films prepared using rf magnetron sputtering and their application to anti-reflection coating

Preparation of TiO₂ and SiO₂ films for optical applications was attempted using conventional rf magnetron sputtering in the sputtering ambient with various O₂/Ar+O₂ ratios and at substrate temperatures between room temperature and 400 °C. X-ray photoelectron spectroscopy (XPS) and optical spectroscopy investigations indicated that oxygen addition in the sputtering ambient was essential for growing TiO₂ films with stoichiometric compositions and good transmittance, while SiO₂ films had a stoichiometric composition of O/Si ratio=2.1-2.2 and were highly transparent in the visible wavelength region, independent of gas composition in the growing ambient. It was also identified from scanning electron microscope (SEM), atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FTIR) measurements that the structural characteristics of both TiO₂ and SiO₂ films were significantly improved with O₂ addition in the sputtering ambient, showing smoother surface morphologies and higher resistances to water absorption when compared with films grown without O₂ addition. Heating of the substrate between 200 and 400 °C considerably increased the refractive index of TiO₂ layers, resulting in dense structures along with an improvement of crystallinity. For optical applications, AR coatings composed of 2-4 multi-layers on glass were designed and manufactured by stacking in turn the SiO₂ and TiO₂ films at room temperature and O₂/Ar+O₂=10%, and the performance of the produced coatings was compared with simulation results.     

Oxidizing gas sensing properties of mesogenic copper octakisalkylthiophthalocyanine chemoresistive sensors

In this study, the effect of oxidizing gases, such as oxygen (O₂), nitrogen dioxide (NO₂), and ozone (O₃), on a liquid-crystalline copper octakisalkylthiophthalocyanine[(C₆S)₈PcCu] thin film was investigated in the temperature range of 25-150 °C. Starting from a chloroform solution of (C₆S)₈PcCu, a jet-spray technique in an inert ambient atmosphere was used to coat the thin film of the compound on to an Interdigital Transducer (IDT) with gold electrodes. The concentration ranges for NO₂ and O₃ exposed to the (C₆S)₈PcCu thin film were 1-10 ppm and 50 ppb-50 ppm, respectively. The response time in NO₂ measurements was observed to be approximately 1 min at room temperature, and it decreased to a few seconds with increasing temperature. A good sensor response of 2000% ppm^− 1 was observed when the sensor was exposed to 1 ppm NO₂ at room temperature. The oxidizing gases were found to be desorbed by annealing the thin film.     

Room-temperature deposition of carbon nanomaterials by excimer laser ablation

Numerous investigators have reported on pulsed laser deposition of carbon nanotubes, mostly using the Nd:YAG laser for ablation. In all cases the depositions have been conducted at high-temperatures and high pressures. Here we report on the deposition of carbon nanostructures at room temperature using a 248 nm excimer laser nm to ablate mixed graphite-nickel/cobalt targets. We find that the formation of the carbon nanomaterials is dependent on the particular ambient gas employed. In O₂ gas, carbon nanotubes and nano-onions are produced. The nanotubes have notably large channel diameters of 100-200 nm and the nano-onion structures are 100-200 nm in diameter, also much larger than previously observed. High-resolution, in-situ, time-resolved emission spectroscopy has been used to follow the production of molecular carbon species such as C₂ and C₃, as well as metals such as Ni or Co in the different ambients employed. Spectral modeling reveals significant differences in the vibrational-rotational temperatures of C₂ spectra in O₂ versus Ar. Mechanistic details of the formation of carbon nanotubes and nano-onions, and in-situ optical emission spectroscopy are described.     

Etch characteristics of magnetic tunnel junction stack with nanometer-sized patterns for magnetic random access memory

Etch characteristics of magnetic tunnel junction (MTJ) stack masked with TiN films were investigated using an inductively coupled plasma reactive ion etcher in Cl₂/Ar and BCl₃/Ar gases for magnetic random access memory. The effect of etch gas on the etch profile of MTJ stacks was examined. As Cl₂ and BCl₃ concentrations increased, the etch slope of etched MTJ stack became slanted and the dimensional shrinkage was observed. A high degree of anisotropic etching of MTJ stacks was achieved using Cl₂/Ar gas at the optimized etch conditions.     

Characterization of N-doped TiO2 films prepared by reactive sputtering using air/Ar mixtures

Nitrogen-doped titanium dioxide (TiO_2 − xN_x) thin films desirable for visible light photocatalysts were prepared by reactive sputtering using air/Ar mixtures. Using air as the reactive gas allows the process to conduct at high base pressures (low vacuum), which reduces substantially the processing time. The obtained films transformed from mixed phases to anatase phase as the air/Ar flow ratio increased. Substitutional doping of nitrogen verified by X-ray photoelectron spectroscopy accounts for the red-shift of absorption edge in the absorption spectra. Anatase TiO_2 − xN_x films could incorporate up to about 7.5 at.% substitutional nitrogen and a maximum of 23 at.% nitrogen was determined in the films with mixed phases. The optical band gaps of the TiO_2 − xN_x films calculated from Tauc plots varied from 3.05 to 3.11 eV and those of the mixed phase ranged from 2.77 to 3.00 eV, which are all lower than that for pure anatase TiO₂ and fall into the visible light regime.     

Etch characteristics of indium zinc oxide thin films in a C2F6/Ar plasma

Dry etching of indium zinc oxide (IZO) thin films was performed using inductively coupled plasma reactive ion etching in a C₂F₆/Ar gas. The etch characteristics of IZO films were investigated as a function of gas concentration, coil rf power, dc-bias voltage to substrate, and gas pressure. As the C₂F₆ concentration was increased, the etch rate of the IZO films decreased and the degree of anisotropy in the etch profile also decreased. The etch profile was improved with increasing coil rf power and dc-bias voltage, and decreasing gas pressure. An X-ray photoelectron spectroscopy analysis confirmed the formation of InF₃ and ZnF₂ compounds on the etched surface due to the chemical reaction of IZO films with fluorine radicals. In addition, the film surfaces etched at different conditions were examined by atomic force microscopy. These results demonstrated that the etch mechanism of IZO thin films followed sputter etching with the assistance of chemical reaction.     

Superconducting and electrical properties of Mg-B structures formed by implantation of magnesium ions into the bulk boron followed by pulse plasma treatment

Superconducting regions of magnesium diboride (MgB₂) on magnesium substrate were formed by the combined methods of ion implantation and transient annealing using three different ion fluences and three different energy density of Ar plasma pulses. The samples were characterized by Rutherford back scattering (RBS) and superconductivity detection techniques. The results of these characterizations are presented and discussed. The highest critical temperature observed T_C=33.8 K.     

ON THE ABSORPTION SPECTRUM OF C60 IN THE VISIBLE SPECTRAL REGION

ABSTRACT

A complete (720–380?nm), strong visible absorption spectrum (OD₅₉₈=1.6) for C₆₀ in a decalin/methylcyclohexane inert matrix at 77?K is reported for the first time. The structure of this spectrum allows one to construct an acceptable approximation to the gas-phase spectrum for this substance at a low temperature. The 0–0 component of the S₀→S₁ transition in the gas phase at a low temperature was estimated to be at about 635?nm.     

Piezoelectric and optical properties of ZnO thin films deposited using various O<Subscript>2</Subscript>/(Ar+O<Subscript>2</Subscript>) gas ratios

In this study, ZnO thin films were fabricated on a Pt(111)/TiO_x/SiO₂/Si substrate using the RF magnetron sputtering method. Then, the effect of the crystallization orientation and microstructure on the piezoelectric and optical properties of the ZnO thin film was investigated for various O₂/(Ar+O₂) gas ratios. When the O₂/(Ar+O₂) gas ratio was 50%, the intensity of the (002) peak corresponding to the preferred orientation of the ZnO thin film was a maximum and the minimum FWHM value of 0.56° was observed. The surface roughness of the ZnO thin film measured using AFM also had a minimum value of 16.43 °C at an O₂/(Ar+O₂) gas ratio of 50%. The piezoelectric characteristics of the ZnO thin film were measured using the pneumatic loading method (PLM) and the corresponding constant had the largest value of 11.9 pC/N at an O₂/(Ar+O₂) gas ratio of 50%. The transmittance of the ZnO thin film obtained from the transmittance curve using a spectrophotometer was slightly greater than 80% in the human visible light region at an O₂/(Ar+O₂) gas ratio of 50%. By using the refractive index data obtained from the transmittance curve and the Sellmeir dispersion relation, we can also predict the refractive index at a wavelength of 400 nm. When the O₂/(Ar+O₂) gas ratio was 50%, the refractive index was 2.043 and, at other gas ratios, the corresponding refractive indices were 2.004∼2.006. The band gap energies of the ZnO thin film were 3.27∼3.33 eV depending on the O₂/(Ar+O₂) gas ratio and were little affected by the variation of the oxygen inflow volume.     

Influence of growth parameters on the dopant configuration of nitrogen-doped graphene synthesized from phthalocyanine molecules

Synthesis of nitrogen-doped graphene (NDG) via chemical vapor deposition (CVD) using phthalocyanine, a solid precursor containing carbon and nitrogen, is reported. The effect of the growth parameters (temperature, time, and carrier gas) on the surface morphology, dopant configuration, and conductivity of the films was studied. The NDG films were synthesized at different substrate temperatures of 1050 °C, 950 °C, and 850 °C for different growth times of 5–15 min in the presence of an Ar?+?H₂ gas mixture. Significantly, pyrrolic-N type defects are observed predominantly after 5 min of growth time. At 1050 °C, pyrrolic N content is around 45.4% after 5 min of growth which decreased to 24.1% after 15 min of growth, while the graphitic-N content increased from 41.2 to 76% at the same time. It is demonstrated that the conversion of pyrrolic type of nitrogen to graphitic nitrogen defects can be arrested by changing the carrier gas from Ar?+?H₂ to Ar. The pyrrolic-N content increased to 64% by changing the gas from Ar?+?H₂ to Ar at 15 min. The electrolyte gated field-effect transistors were fabricated using the obtained films, and dopant-dependent mobility was observed. The mobility for pyrrolic-N-dominated film is 13.6 cm² V^?1 s^?1 increasing to 62.8 cm² V^?1 s^?1 for graphitic-N-dominated film.

     

Novel properties of AZO film sputtered in Ar+H2 ambient at high temperature

AZO (ZnO:Al) transparent conductive thin film was prepared by RF magnetron sputtering with a AZO (98 wt% ZnO 2 wt% Al₂O₃) ceramic target in the same Ar+H₂ ambient at different substrate temperatures ranging from 100 to 300 °C. The minimum resistivity of AZO films was 7.9×10⁻⁴ Ω cm at the substrate temperature of 200 °C. The average transmission in the visible rang was more than 90%. Scanning electron microscopy and XRD analyses showed that the surface morphology of the AZO samples altered with the increasing of the substrate temperature. AZO film prepared at 200 °C in the pure Ar ambient was also made as comparison about the resistivity, carrier concentration and the average crystallite size. The resistivity became about 3 times higher. The carrier concentration became lower and the average crystallite size was smaller.     

Carbon nanotubes synthesis in microwave plasma torch at atmospheric pressure

Well aligned multi-walled carbon nanotubes were synthesized at atmospheric pressure using a microwave plasma torch on silicon substrates with silicon oxide buffer layer and catalyst overlayer in the mixture of argon, hydrogen and methane. Iron or nickel was used as catalysts. The optimum substrate temperature for the deposition on Si/SiO₂/Fe substrates was about 970 K. In this case SEM micrographs of the deposits revealed a presence of vertically aligned nanotubes with the diameters around 15 nm. TEM micrographs showed a presence of amorphous carbon particles in the samples and some defects in the wall structure of the produced nanotubes. In Raman spectra two peaks at 1332 and 1584 cm⁻¹ were observed. The CNTs were also synthesized on the substrates without SiO₂ buffer layer but their quality was lower. The synthesis with Ni instead of Fe catalyst required lower temperature and the alignment of the nanotubes was worse. The deposition process was monitored by optical emission spectroscopy. Atomic lines of hydrogen and argon, an emission of CN due to a presence of nitrogen impurities from atmosphere, a weak molecular band of CH and strong C₂ emission were detected in the spectra.