Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of IG/ID is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles

Development of nanostructured carbon thin films

In this thesis, experimental investigation of the growth mechanism of unique nanostructured carbon films and their properties as well as different fabrication techniques of these films is reported. Using experimental results, the presented mechanisms have been evaluated and developed. This work has enabled a larger range of deposition parameters to create user-specific nanostructured carbon films. The parameters examined include heat, depositing ion energy, plasma density and post treatments such as laser and thermal annealing. Electrical, thermal and field emission properties of nanostructured carbon films have also been studied. First, the effects of deposition temperature and ion energy on the microstructure of the carbon films have been investigated. To do this, the microstructure of carbon films deposited at temperature range of 25 to 6000C and substrate bias range of 25 to 600 V have been studied by plan view and cross section transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and Raman spectroscopy. It is found that at low deposition temperatures ( 1500C) the microstructure of the film depends on the substrate bias. at low substrate biases (lower than 400V) the films are amorphous in the microstructure. Increasing the bias to 600V results in formation of preferred oriented nanocrystals in the microstructure. This is attributed to the formation of high temperature thermal spikes due to impinging of high energy ions to the growing film. Increasing the substrate temperature to 400 and 6000C leads to formation of preferred oriented nanocrystals even at floating substrate bias. The nature of the nanocrystals however depends on the applying bias. Low substrate biases (lower than 600V) results in the formation of graphitic like nanocrystals while at 600V tubular nanostructures are formed. This is due to higher formation enthalpy of tubular carbon structures compare to graphene sheets. In order to study the effect of plasma parameters, carbon films were prepared under two different plasma densities (2.5 and 12.5 mA/cm2) and different substrate biases (25 to 500 V). It is found that by applying high ion density plasma, nanocrystals are formed at room temperature even at low substrate biases (300 V). Meanwhile, decreasing the ion density increases the threshold ion energy for graphitization. More importantly it is experimentally shown that the nature of the nanocrystals strongly depends on the depositing ion energies. High ion energy (higher than 500 eV) results in formation of tubular nanostructures while lower ion energies (300 to 500 eV) results in formation of graphitic nanostructures. Stability of different nanostructures have been discussed in terms of the thermal spike temperature. The experimental results of the formation of different nanostructures have been proven by molecular dynamics simulations. Separately, the properties of textured nanostructure carbon films were also studied. The first property investigated was the electrical conductivity of the films. It is found that formation of preferred oriented nanocrystals results in significant increase in the conductivity. The conduction in the amorphous films is limited through Poole-Frenkel mechanism. Electron will hoop between the conductive sp2 sites. Therefore, the conductivity of the amorphous films is controlled by the amount, size and distribution of sp2 bonded nanocrystals embedded in the amorphous sp3 matrix. Formation of preferred oriented nanocrystals results in the formation of continuous sp2 bonded channels which enhances the conductivity by three orders of magnitude. In order to induce the nanostructures locally a local post deposition treatment is needed. Hence, the application of the laser annealing has been studied. To do this, a wide range of initial a-C structures (from ta-C to high sp2 content a-C films) have been irradiated by a KrF Excimer laser with pulse width of 23 ns. The structural changes have been studied by Raman spectroscopy, TEM and EELS. It has been shown that the behavior of carbon films upon laser irradiation strongly depends on the initial bonding structure of the films. Using high sp2 content a-C film as the initial structure, results in the formation of graphitic nanocrystals at moderate laser energies (higher than 360 mJ/cm2). However, ta-C films are stable even at higher laser energies. Field emission and thermal conductivity of textured carbon films have also been investigated. It is found that formation of conductive sp2 channels throughout the thickness of the films which is achieved by the formation of texture in the microstructure, affects the emission threshold field significantly. This is mainly due to simultaneous activation of two field enhancement mechanism namely the presence of highly conductive phase (sp2 bonded filaments) embedded in an insulative (amorphous sp3) matrix and the formation of high aspect ratio graphitic filaments. As such, the threshold emission filed on an a-C film decreased from 12 to 3.5 V/μm by a single nsec laser irradiation at 462.5 mJ/cm2. It is also shown that thermal conductivity of carbon films depend on the microstructure of the films. Pulsed photothermal reflectance (PPR) has been used to study the thermal conductivity of carbon films deposited at different temperatures (amorphous and nanocrystalline carbon films). It is found that formation of highly conductive graphitic nanostructures perpendicular to the substrate, increases the thermal conductivity. Compare to thermal conductivity of a-C films (~ 1 W/m.K) textured carbon films show an order of magnitude ( upto 17 W/m.K) increase in thermal conductivity.DOCTOR OF PHILOSOPHY (EEE

Thickness dependency of field emission in amorphous and nanostructured carbon thin films

Thickness dependency of the field emission of amorphous and nanostructured carbon thin films has been studied. It is found that in amorphous and carbon films with nanometer-sized sp2 clusters, the emission does not depend on the film thickness. This further proves that the emission happens from the surface sp2 sites due to large enhancement of electric field on these sites. However, in the case of carbon films with nanocrystals of preferred orientation, the emission strongly depends on the film thickness. sp2-bonded nanocrystals have higher aspect ratio in thicker films which in turn results in higher field enhancement and hence easier electron emission.Published versio

Electrowetting control of Cassie-to-Wenzel Transitions in superhydrophobic carbon nanotube-based nanocomposites

The possibility of effective control of the wetting properties of a nanostructured surface consisting of arrays of amorphous carbon nanoparticles capped on carbon nanotubes using the electrowetting technique is demonstrated. By analyzing the electrowetting curves with an equivalent circuit model of the solid/liquid interface, the long-standing problem of control and monitoring of the transition between the "slippy" Cassie state and the "sticky" Wenzel states is resolved. The unique structural properties of the custom-designed nanocomposites with precisely tailored surface energy without using any commonly utilized low-surface-energy (e.g., polymer) conformal coatings enable easy identification of the occurrence of such transition from the optical contrast on the nanostructured surfaces. This approach to precise control of the wetting mode transitions is generic and has an outstanding potential to enable the stable superhydrophobic capability of nanostructured surfaces for numerous applications, such as low-friction microfluidics and self-cleaning

Wettability, nanoscratch resistance and thermal stability of filtered cathodic vacuum arc grown nitrogenated amorphous carbon films

International audienceComposition, structure, surface energy, nanoscratch resistance and thermal stability of nitrogenated amorphous carbon films grown by filtered cathodic vacuum arc (FCVA) are studied in this paper. X-ray photoelecti-on spectroscopy and electron energy loss spectroscopy studies reveal that by controlling the nitrogen flow rate and substrate bias carbon films with different bonding structures and composition are formed. Higher nitrogen flow rate results in higher nitrogen content of the film and the stability of C equivalent to N bonds. Increasing the nitrogen content of the films (0 to 16 at.%) increases the polar surface energy (10 to 22 mJ/m(2)) of the films while the dispersive surface energy does not change significantly. Thermal stability of the films strongly depends on the composition and bonding structure. The films deposited at higher substrate bias "(300 V) and containing higher nitrogen content undergo graphitization at lower annealing temperatures. There is no significant difference in the scratch resistance of the films at small scratch loads (up to 35 mu N). Further increase in the scratch load results in larger scratch depth in the film deposited at high nitrogen flow rate (40 sccm)

Microstructure and electrical properties of in-situ annealed carbon films

The microstructure and electrical properties of in-situ annealed carbon films is studied in this paper. The structure of the films is studied by transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy. The microstructure of the films strongly depends on the deposition temperature for the films deposited at high temperatures (higher than 400°C). However, at low temperatures the substrate bias is the other crucial factor which governs the microstructure of the film. Electrical conductivity of the film strongly depends on the formation of preferred orientation in the microstructure of the films.Accepted versio

Superhydrophobic carbon nanotube/amorphous carbon nanosphere hybrid film

Fabrication of superhydrophobic surfaces has been widely investigated due to their wide range of applications. Here, synthesis of self-assembled aligned carbon nanotubes (ACNT)/amorphous carbon (a-C) nanosphere hybrid film is reported. Carbon plasma produced by FCVA was used to deposit a-C nanospheres on the ACNT films fabricated by PECVD. The superhydrophobic properties of the surface was investigated by static contact angle (CA) measurement. It is found that the surface morphology of the film which depends on the size of the a-C nanospheres, has a great influence on the hydrophobic properties of the surface. The hydrodynamic properties of the surface is discussed in terms of both Cassie and Wenzel mechanisms. The microstructure of the films is also investigated by XPS and HRTEM. It is shown that the bombardment of the CNTs with high energy carbon ions will damage the crystalline structure of the CNT walls as well

Highly conductive aligned carbon film for interconnect application

We demonstrate here an amorphous carbon film with a highly conductive transition phase for interconnect application. The uniform orientation of graphite-like sheet provides an effective carriers transport channel, thus significantly improves the conductivity of the amorphous carbon film. The electrical properties of the aligned carbon film are investigated using Kelvin structure. Our results show that the resistivity of aligned carbon film is comparable with that of Cu, and the aligned carbon film holds promise for future interconnect application.Accepted versio

Novel solution for high-temperature dielectric application to encapsulate high-voltage power semiconductor devices

Traditional semiconductor packaging techniques and materials have been working well for conventional Si devices, which usually operate at temperatures up to 175 °C. As the operating temperature increases, these techniques exhibit failures such as bulk flows, volume shrinkage, brittleness and subsequent cracking, and deterioration of dielectric strength. For the new wide-bandgap power devices, which work at voltages as high as 1200 V and junction temperatures as high as 250 °C, there is currently no known dielectric material to encapsulate and protect the active devices. This paper summarizes novel solutions for high-temperature dielectric materials for encapsulating high-power semiconductor devices without any dielectric breakdown and also without introducing excessive thermal, electrical, and mechanical stresses to the encapsulated devices. Polymer dielectric candidates investigated in this paper include cyanate ester-based resin and silicone-based resin. In addition, conformal coating approaches that include alumina deposited by atomic layer deposition (ALD) technique and tetrahedral amorphous carbon (ta-c) deposited by filtered cathodic vacuum arc technique were evaluated in this paper. Dielectric strength performance of the material combinations with respect to temperatures was evaluated. Among the polymer encapsulants, a silicone resin with silica fillers was determined to be the prospective candidate. Breakdown voltage and leakage current of the silicone-based encapsulant with and without the conformal coating was measured by breakdown tests. It was determined that the ta-c conformal coating deteriorates the dielectric performance of the encapsulant, while the alumina thin film deposited by ALD approach reduces the leakage current of the encapsulation material and also increases the breakdown voltage of the silicone encapsulant. Thus, the combination of alumina thin film deposited by ALD approach along with the silicone encapsulant is recommended for this application, involving high temperature and high voltage.Agency for Science, Technology and Research (A*STAR)Accepted versionThis work was supported by the A*STAR Aerospace Consortium

Microstructure and through-film electrical characteristics of vertically aligned amorphous carbon films

The microstructure and electrical properties of in-situ annealed carbon films is studied in this paper. In-situ annealing (150 °C to 600 °C) was done during the deposition of carbon films with −300 V substrate bias. Transmission electron microscopy and two points electrical probing studies were performed and the deduced transition for vertical orientated graphitic planes occurs at temperatures above 400 °C. The microstructure of the films strongly depends on the deposition temperature of the films (room temperature, 400 °C and 600 °C). Electrical conductivity of the film strongly depends on texturing due to the formation of preferred orientation in the vertical direction. The vertically orientated carbon (VOC) sheet provides effective nanochannels for electron transport, thus significantly improves the electrical properties of the annealed film.Ministry of Education (MOE)The author would like to thank MoE Tier II funding ARC 13/08