Synthesis of One-Dimensional And Two-Dimensional Carbon Based Nanomaterials

Particular physical and chemical properties of carbon based nanomaterials (CBNs) have promised and exhibited great applications in manufacturing various nanodevices such as electron field emitters, sensors, one-dimensional conductors, supercapacitors, reinforcing fibres, hydrogen storage devices, and catalyst support for fuel cells electrodes. Despite these amazing technical progresses, many challenges still remain in the development of synthesis methods suitable for commercial applications and fabricating novel functional nanostructures with complex architecture. In this Ph.D. thesis, one-dimensional (1D), two-dimensional (2D) carbon nanostructures, and 1D/2D hybrid of carbon nanostructures have been synthesized using various chemical vapour deposition (CVD) methods. The objective of this work is to explore the potential of various CVD methods, including specially-designed CVD techniques, such as modified spray pyrolysis, plasma enhanced CVD, and magnetron sputtering deposition. By making use of these innovative methods, high density regular and nitrogen-doped nanotubes, graphite nanosheets and assemblies have been successfully obtained on conducting and semiconducting substrates. For the modified spray pyrolysis method, systematic investigation of regular carbon nanotubes (CNTs) was conducted in terms of optimizing various experimental parameters such as hydrocarbon source, temperature, and catalyst in order to control the quality and structure of CBNs. Doping of nitrogen into carbon nanotubes was also systematically studied to enhance their electrical and mechanical properties. Interestingly, a novel structure of multi-branched nitrogen doped CNTs has been achieved by this modified spray pyrolysis method. By employing the plasma assisted CVD/sputtering hybrid system, selective growth of single and few walled CNTs have been realized. The device has also been able to produce 2D carbon nanostructures of nanosheets and a hybrid of nanosheets suspended on vertical aligned CNTs. Based on the magnetron sputtering deposition method, carbon nanowalls have been synthesized without any catalyst addition. Morphology, microstructure, and vibration properties of the CBNs were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Carbon nanomaterials, grown in high densities on conducting and semiconducting substrates, promise great potential in building various nanodevices with different electron conducting requirements. In addition, CBNs provide a very high surface area for the support of platinum particles for use in hydrogen fuel cell electrodes

Compilation on Synthesis, Characterization and Properties of Silicon and Boron Carbonitride Films

During the last years the interest in silicon and boron carbonitrides developed remarkably. This interest is mainly based on the extraordinary properties, expected from theoretical considerations. In this time significant improvements were made in the synthesis of silicon carbonitride SiCxNy and boron carbonitride BCxNy films by both physical and chemical methods. In the Si–C–N and B-C-N ternary systems a set of phases is situated, namely diamond, SiC, -Si3N4, c-BN, B4C, and -C3N4, which have important practical applications. SiCxNy has drawn considerable interest due to its excellent new properties in comparison with the Si3N4 and SiC binary phases. The silicon carbonitride coatings are of importance because they can potentially be used in wear and corrosion protection, high-temperature oxidation resistance, as a good moisture barrier for high-temperature industrial as well as strategic applications. Their properties are low electrical conductivity, high hardness, a low friction coefficient, high photosensitivity in the UV region, and good field emission characteristics. All these characteristics have led to a rapid increase in research activities on the synthesis of SiCxNy compounds. In addition to these properties, low density and good thermal shock resistance are very important requirements for future aerospace and automobile parts applications to enhance the performance of the components. SiCxNy is also an important material in micro- and nano-electronics and sensor technologies due to its excellent mechanical and electrical properties. The material possesses good optical transmittance properties. This is very useful for membrane applications, where the support of such films is required (Fainer et al., 2007, 2008; Mishra, 2009; Wrobel, et al., 2007, 2010; Kroke et al., 2000). The structural similarity between the allotropic forms of carbon and boron nitride (hexagonal BN and graphite, cubic BN and diamond), and the fact that B-N pairs are isoelectronic to C-C pairs, was the basis for predictions of the existence of ternary BCxNy compounds with notable properties (Samsonov et al., 1962; Liu et al., 1989; Lambrecht & Segall, 1993; Zhang et al., 2004). This prediction has stimulated intensive research in the last 40 years towards the synthesis of ternary boron carbonitride. BCxNy compounds are interesting in both the cubic (c-BCN) and hexagonal (h-BCN) structure. On the one hand, the ..

Transparent conducting oxides and other functional thin films grown via aerosol assisted chemical vapour deposition

This thesis describes the preparation and characterisation of different functional thin films, with the main focus being transparent conducting oxide (TCO) thin films including both n-type and p-type, but also other functional thin films (such as catalytic thin films used in the oxygen evolution reaction (OER)) grown via aerosol assisted chemical vapour deposition (AACVD). The main aim to this work was to discover and investigate more suitable functional thin films (TCOs and catalytic materials) via the film preparation method, AACVD. The synthetic route of all the functional thin films used in this thesis is AACVD, which is one specialized form of CVD that is cost effective and easily scalable, operating at ambient pressure. In this thesis, Mo-, P- and B-doped zinc oxide (ZnO) thin films have been investigated as n-type TCOs, with all films showing low resistivities of 2.6 × 10-3 Ω.cm, 6.0 × 10-3 Ω.cm and 5.10 × 10-3 Ω.cm, respectively. The optical properties also reached over 75% as transmittance in the visible area for all doped thin films. Boron phosphide (BP) thin films were synthesized and investigated as a p-type TCO, although the results indicated that BP may not be an ideal p-type material and further doping investigations were not investigated. P-doped molybdenum disulfide (MoS2) thin films have been considered as potential catalytic materials in the OER area. The MoS2 thin films with 1 mol.% P doping displayed superior catalytic performance of OER process with lowest overpotential at 319 mV for 10 mA cm-2 as current density and 173 mV for 10 mA mg-1 as current density, respectively, in 1M KOH medium. Moreover, the mass activity was also high at 1000 A g-1 with small overpotential at 450 mV, which suggested the doping method could improve the properties of thin films through AACVD

Synthesis of N-doped broken hollow carbon spheres and inorganic-organic hybrid perovskite materials for application in photovoltaic devices

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for degree of Master of Science in ChemistryThe mandate for renewable energy sources to replace the current reliance on fossil fuels as a primary energy source has recently attracted a lot of research interest. The research has also focussed on bringing the technologies that take into consideration the goal of reducing environmental pollution. Consequently, approaches using photovoltaic (PV) technologies have been a promising arena to tackle the problem facing energy sources. Recently, more focus has been placed on improving the power conversion efficiency (PCE) of PV devices, such as organic and/or organic-inorganic hybrid perovskite solar cells. Therefore, in this work two different materials were applied in two independent PV devices, namely organic and/or organic-inorganic hybrid perovskite solar cells. One study employed nitrogen doped broken hollow carbon spheres (N-bHCSs), with an aim of enhancing the electronic properties of the P3HT:PCBM active layer of an organic photovoltaic (OPV) solar cell. N-bHCSs were successfully synthesized using a horizontal chemical vapour deposition method (H-CVD) employing a template-based method and the carbon was doped using in-situ and ex-situ doping techniques. Pyridine, acetonitrile and toluene were used as both carbon and nitrogen precursors. The dispersity of the SiO2 spheres (i.e. templates) was found to play a role on the breakage of the N-bHCSs. Incorporation of the N-bHCSs into the P3HT:PCBM active layer was found to enhance the charge transfer and this led to less recombination of photogenerated charges in the interface between the donor and acceptor. The current-voltage (I-V) characteristics of the ITO/PEPOT:PSS/P3HT:PCBM:N-bHCSs/Al solar cell devices revealed an increased chargetransport distance due to increased electron density by n-type doping from the N-bHCSs. The second study employed the organic-inorganic hybrid perovskite (CH3NH3PbI3) material as a light harvesting layer in an ITO/PEDOT:PSS/CH3NH3PbI3/PC6BM/Al solar cell device. Initially, the device parameters were optimised to obtain the best performing device. These include parameters such as the degradation of the hybrid film as a function of time and air exposure. A rapid degradation was seen on the device after 24 h of air exposure which was accompanied by the decrease in the PV performance of the device. The degradation was visually seen by the formation of crystal grains (i.e. “islands”) on the perovskite film.GR201

Carbon-Based Materials

New carbon materials with improved mechanical, electrical, chemical, and optical properties are predicted and considered to be very promising for practical application. Carbon-based materials in the form of films, fabrics, aerogels, or microstructural materials are known for their large surface areas and pore volumes, light weight, and a great variety of structural morphology. Such unique structures can then be employed for a variety of purposes, for example, the production of new electronic devices, energy storage, and the fabrication of new materials. Nowadays, clear understanding of carbon materials via several examples of synthesis/processing methodologies and properties characterization is required. This Special Issue, “Carbon-Based Materials”, addresses the current state regarding the production and investigation of carbon-based materials. It consists of 13 peer-reviewed papers that cover both theoretical and experimental works in a wide a range of subjects on carbon structures

Characterization of low pressure chemically vapor deposited Boron Nitride films as low dielectric constant materials

Boron nitride thin films were synthesized on Si and quartz wafers by low pressure chemical vapor deposition using borane triethylamine complex and ammonia as precursors. The films were processed at 400°C, 475°C and 550°C at a constant pressure of 0.5 Torr and at different precursor flow ratios. The films deposited were uniform, amorphous and the composition of the films varied with deposition temperature and precursor flow ratios. The thickness of the film increased with increasing flow ratio, but, decreased with increasing temperature. The stresses in the film were either mildly tensile or compressive. The least dielectric constant for the films that could be attained was 2.73 at 550°C and at high flow ratios of NH3/TEAB (50/1). Thus, stoichiometric boron nitride films tend to have a lower dielectric constant. The limitation of attaining lower values could be due to the presence of carbon as an impurity in the film and the presence of mobile charge carriers in the films as well as at the substrate-film interface as seen from the capacitance-voltage characteristics

Status and applications of diamond and diamond-like materials: An emerging technology

Recent discoveries that make possible the growth of crystalline diamond by chemical vapor deposition offer the potential for a wide variety of new applications. This report takes a broad look at the state of the technology following from these discoveries in relation to other allied materials, such as high-pressure diamond and cubic boron nitride. Most of the potential defense, space, and commercial applications are related to diamond's hardness, but some utilize other aspects such as optical or electronic properties. The growth processes are reviewed, and techniques for characterizing the resulting materials' properties are discussed. Crystalline diamond is emphasized, but other diamond-like materials (silicon carbide, amorphous carbon containing hydrogen) are also examined. Scientific, technical, and economic problem areas that could impede the rapid exploitation of these materials are identified. Recommendations are presented covering broad areas of research and development

Synthesis and characterization of LPCVD SiC films using novel precursors

A unique low pressure chemical vapor deposition (LPCVD) process has been developed to synthesize amorphous and crystalline SiC films using environmentally benign chemicals. The interrelationships governing the process variables, compositions and select properties of the resulting films were established. Such films can be used to produce high quality mask membrane for x-ray lithography. These films can also be used in fabricating high power electrical devices, and hetrojunction devices in conjunction with silicon. Amorphous SiC films were synthesized using a single precursor, ditertiarybutylsilane, at temperatures below 850°C. Compositional analysis performed on these deposits revealed that, in the deposition temperature range of 625 to 750°C, the composition of the deposits changed progressively from slightly silicon rich (55% Si) to slightly carbon rich (51%C). Above 750°C, there was a rapid increase in the carbon content from the near stoichiometric value to about 75%-C at 850°C. The stoichiometric films exhibited high stress values of 700 ± 50 MPa. Attempts to reduce the stress values resulted in films with excess carbon content of about 60%-C. From the high frequency C-V characterization, the dielectric constant for these films was estimated to be 10.1 ± 0.5. Temperature bias stressing studies revealed a trapped charge density of 0.869 X 107 CM-2 within the bulk. Crystalline silicon carbide films were grown on silicon substrates using dichlorosilane and acetylene as precursors, in the temperature range of 950°C to 1050 C. The carbon content in the film was found to be increasing with the deposition temperature, when the flow ratio of precursors was one. The carbon composition was also found to be sharply dependent on acetylene flow, for constant deposition temperature and pressure. Stoichiometric films were achieved for dichlorosilane to acetylene flow ratio of 4:1. X-ray diffraction studies confirmed the growth of β-SiC with orientation in all the cases. The voltage-current relationship for Si-film-metal structure showed a diode behavior with an ideality factor of 4.03 in the diffusion current dominating regime

Hot Filament Chemical Vapor Deposition of Semiconducting Boron Carbide Thin Films

A highly efficient, low-power, compact thermal neutron detection system with excellent gamma-ray discrimination is desired for a number of applications. 10B [boron- 10] has a large cross section for thermal values and a Q-value of 2.78 MeV. For this reason, investigations into boron carbide, boron nitride, and boron phosphide semiconductor neutron detectors are underway. Because boron carbide has the highest fraction of boron of the three, it holds the highest potential. With this in mind, a hot filament chemical vapor deposition (HFCVD) system was designed and built in order to grow thin films of boron carbide onto n-type silicon substrates. Deposition was accomplished via the thermal decomposition of B2H6 [diborane] and CH4 [methane]