2,313 research outputs found

    Nitrogen-Doped Diamond:A Thermionic Investigation on the Effects of Lithium Oxide Termination

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    This project aims to investigate the emission currents produced from nitrogen-doped dia- mond samples terminated with oxygen and lithium, to develop our understanding of such a material for use as an emitter surface in a thermionic energy converter (TEC). Thermionic devices offer clean, renewable energy generation from a device with no moving parts thus a large potential for device efficiency, such a device could be revolutionary for electricity gen- eration using solar power, or for clean hydrogen production from electrolysis. TECs could also be used for space-based solar power or energy generation for space travel due to the high physical and radiation hardness of diamond. For an effective TEC device, the semicon- ductor emitter surface must exhibit a negative electron affinity (NEA) for electron emission, as well as a low work function (WF) to reduce the operating temperatures of such a thermal device. Nitrogen is a deep-level electron donor when used as a diamond dopant, it easily incorpo- rates into the diamond lattice through substitutional sites. Lithium terminations have been studied both theoretically and experimentally to both induce an NEA and to reduce the work function by inducing dipoles on the surface of the diamond films, altering the semi- conductor’s band structure through electrostatic band bending.1,2,3 Oxygenation has shown to increase the bond strength of lithium to the surface as well as that of the surface dipoles, further reducing the WF.2,4,2 Diamond films have been grown using chemical vapour deposition (CVD) in nitrogen-rich environments, the effects of changing nitrogen flow rates within the gaseous growth mixture have been investigated in terms of sample quality and electronic properties of the resulting sample. Oxygenation will be used to increase the stability of the terminated lithium surface as well as to further reduce the resulting WF, these terminations have been carried out by UVO-cleaning treatments and thermal evaporation respectively. X-ray photoemission spectroscopy (XPS) has been used to identify surface contaminants as well as to calibrate lithium deposition rates. UV photoemission spectroscopy (UPS) has been used to obtain electron energy distributions for the samples under test to extract estimations for the valence band maximum (VBM) and the WF of the surfaces. The lowest WF achieved was estimated at 1.79eV. Simulated solar heating of the samples by an IR laser, employing the use of a diamond-like-carbon (DLC) absorber surface, was used to measure emission currents from the sample. Emission currents were obtained for the main samples under test as well as for the common NEA-imparting hydrogen termination as a comparison for the other samples due to the well-documented properties it bestows. The largest emission cur- rent achieved for an oxygenated-lithiated surface was 0.016mA at 700◦C, although this value was lower than expected, emission was very stable over all six cycles. This work demon- strates the great potential of lithium-oxide nitrogen-doped diamond emitters

    INVESTIGATION OF DIAMOND COATINGS ON IRON BASED MATERIALS BY MICROWAVE PLASMA CHEMICAL VAPOR DEPOSITION

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    Diamond thin films on Fe based materials (ferrous alloys) for the purpose of improving their surface properties such as wear and corrosion resistance have been investigated. There are main barriers restricting the quality and adhesion of diamond coatings on Fe based materials. Firstly, the incubation time of diamond nucleation is long due to the high solubility of carbon in Fe. Secondly, graphite soot forms before diamond nucleation due to the catalytic effect of Fe for formation of graphitic carbon. Thirdly, high internal stress remains at the interface which is induced by the large difference in the thermal expansion coefficients of diamond and most of the Fe based materials. Surface modification and interlayers are two important approaches to overcome these problems. In this work, the effect of Cr content in Fe-Cr alloys on diamond nucleation and growth is being studied in order to clarify the mechanisms of Cr in diamond deposition. Furthermore, in order to enhance the adhesion and quality of diamond coatings, Al based interlayers are being investigated on ferrous alloys. Fe-Cr alloys (with 20~80 wt.% Cr) were exposed to a CH4-H2 mixture in a microwave plasma enhanced chemical vapor deposition (MPCVD) reactor. Severe metal dusting and carburization were observed on the alloys with low Cr content and diamond did not nucleate on those alloys until a graphite intermediate layer had been formed, which takes a long incubation time. Increasing Cr concentration in the Fe-Cr alloys promotes the formation of a Cr carbide buffer layer, which inhibits metal dusting and the formation of graphite soot. Consequently, diamond nucleation and growth can be greatly enhanced, and continuous diamond films with enhanced adhesion have been deposited on the Fe-80Cr alloys. Al based interlayers including Al and Al/AlN interlayers were deposited on ferrous alloys (SS316 and Kovar: FeNiCo) to enhance diamond deposition. The deposition was carried out in a microwave plasma chemical vapor deposition (MPCVD) reactor using a CH4-H2 mixture. The obtained samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, synchrotron-based X-ray absorption spectroscopy (XAS) and indentation testing. The results show that a single Al layer can effectively suppress the formation of graphite at the interface and the inward diffusion of carbon into Fe based substrates, and thereby enhances diamond nucleation and growth. The dual layers of Al/AlN can further enhance the adhesion of diamond coatings comparing with the single Al interlayer

    Thermionic emission properties of novel carbon nanostructures.

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    Materials with low work function values (\u3c 2 eV) are highly in demand for low temperature thermionic electron emission, which is a key phenomenon for waste heat recovery applications. Here we present the study of the thermionic emission of the hybrid structure phosphorus, (P) doped diamond nano crystals grown on conical carbon nanotubes (CCNTs). The CCNTs provide the conducting backbone for the P-doped diamond nanocrystals. In the first part of this thesis thermionic emission properties of conical carbon nanotubes (CCNTs) grown on platinum wires and planar graphite foils were investigated. The work function (Φ) values extracted from the thermionic emission data range from 4.1 to 4.7 eV. The range of Φ values is attributed to the morphological characteristics, such as tip radius, aspect ratio, density, and wall structure of CCNTs. The observed lower values for Φ are significantly smaller than that of multi-walled carbon nanotubes (MWNTs). The reduced Φ values are attributed to field penetration effect as a result of the local field enhancement from these structures having high aspect ratio and an excellent field enhancement factor. The high amplification of the external field at the apex of the nanostructures is capable of reducing both the barrier height and the width, in turn contributing to the improved emission current at lower temperatures. The ultraviolet photoemission spectroscopy data of CCNTs grown on Pt wires are in reasonable agreement with the thermionic emission data. In the next part of the thesis we present work function reduction of phosphorus (P) doped (i) diamond nanocrystals grown on conical carbon nanotubes (CCNTs) and (ii) diamond films grown on silicon substrates. Thermionic emission measurements from phosphorus doped diamond crystals on CCNTs resulted in work function value of 2.23 eV. The reduced work-function is interpreted as due to the presence of the surface states and midband-gap states and no evidence for negative electron affinity was seen. However, Ultraviolet photo-spectroscopy studies on phosphorus doped diamond films yielded a work function value of ~1.8 eV with a negative electron affinity (NEA) value of 1.2 eV. Detailed band diagrams are presented to support the observed values for both cases. In addition we determined the work function values of nanocrystalline P doped diamond films grown on W foil to be significantly lower, 1.0- 1.33 eV compared to the hybrid structure and polycrystalline film on Si substrates. We studied tungsten (W) nanowires as an alternative material in place of CCNT as the supporting and conducting channel for P doped diamond crystals in a new hybrid structure. We described the process of fabrication of arrays of vertical W nanowires by microwave plasma treatment and synthesis of P doped nanocrystalline diamond on top of the reduced W nanowires. Thermionic emission measurements from the alternative hybrid structure resulted in high value of the work function ~ 5.1 eV

    Laser-Spark Multicharged Ion Implantation System ‒ Application in Ion Implantation and Neural Deposition of Carbon in Nickel (111)

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    Carbon ions generated by ablation of a carbon target using an Nd:YAG laser pulse (wavelength λ = 1064 nm, pulse width τ = 7 ns, and laser fluence of 10-110 J/cm2) are characterized. Time-of-flight analyzer, a three-mesh retarding field analyzer, and an electrostatic ion energy analyzer are used to study the charge and energy of carbon ions generated by laser ablation. The dependencies of the ion signal on the laser fluence, laser focal point position relative to target surface, and the acceleration voltage are described. Up to C4+ are observed. When no acceleration voltage is applied between the carbon target and a grounded mesh in front of the target, ion energies up to ~400 eV/charge are observed. The time-of-flight signal is analyzed for different retarding field voltages in order to obtain the ion kinetic energy distribution. The ablation and Coulomb energies developed in the laser plasma are obtained from deconvolution of the ion time-of-flight signal. Deconvolution of the time-of-flight ion signal to resolve the contribution of each ion charge is accomplished using data from a retarding field analysis combined with the time-of-flight signal. The ion energy and charge state increase with the laser fluence. The position of the laser focal spot affects the ion generation, with focusing ~1.9 mm in front of the target surface yielding maximum ions. When an external electric field is applied in an ion drift region between the target and a grounded mesh parallel to the target, fast ions are extracted and separated, in time, due to increased acceleration with charge state. However, the ion energy accelerated by the externally applied electric field is less than the potential drop between the target and mesh due to plasma shielding. By coupling a spark discharge into a laser-generated carbon plasma, fully-stripped carbon ions with a relatively low laser pulse energy are observed. When spark-discharge energy of ~750 mJ is coupled to the carbon plasma generated by ~50 mJ laser pulse (wavelength 1064 nm, pulse width 8 ns, intensity 5 × 109 W/cm2), enhancement in the total ion charge by a factor of ~6 is observed, along with the increase of maximum charge state from C4+ to C6+. Spark coupling to the laser plasma significantly reduces the laser pulse energy required to generate highly-charged ions. Compared to the laser carbon plasma alone, the spark discharge increases the intensity of the spectral emission of carbon lines, the electron density ne, and the electron temperature Te. The effective ion plasma temperature associated with translational motion along the plume axis Tieff is calculated from the ion time-of-flight signal. Carbon laser plasma generated by an Nd:YAG laser (wavelength 1064 nm, pulse width 7 ns, fluence 4-52 J/cm2) is studied by optical emission spectroscopy and ion time-of-flight. Up to C4+ ions are detected with the ion flux strongly dependent on the laser fluence. The increase in ion charge with the laser fluence is accompanied by observation of multicharged ion lines in the optical spectra. The time-integrated electron temperature Te is calculated from the Boltzmann plot using the C II lines at 392.0, 426.7, and 588.9 nm. Te is found to increase from ~0.83 eV for a laser fluence of 22 J/cm2 to ~0.90 eV for 40 J/cm2. The electron density ne is obtained from the Stark broadened profiles of the C II line at 392 nm and is found to increase from ~2.1x1017 cm-3 for 4 J/ cm2 to ~3.5 x 1017 cm-3 for 40 J/cm2. Applying an external electric field parallel to the expanding plume shows no effect on the line emission intensities. Deconvolution of ion time-of-flight signal with a shifted Maxwell-Boltzmann distribution for each charge state results in an ion temperature Ti ~4.7 and ~6.0 eV for 20 and 36 J/cm2, respectively. Carbon ion emission from femtosecond laser ablation of a glassy carbon target is studied. A Ti:sapphire laser (pulse duration τ ~150 fs, wavelength λ = 800 nm, laser fluence F ≤ 6.4 J/cm2) is used to ablate the carbon target while ion emission is detected by a time-of-flight detector equipped with a three-grid retarding field analyzer. A strong effect of the laser pulse fluence on the yield of carbon ions is observed. Up to C6+ ions are detected. The carbon time-of-flight ion signal is fit to a shifted Maxwell-Boltzmann distribution and used to extrapolate the effective plasma ion temperature Tieff = 6.9 eV. Applying an external electric field along the plasma expansion direction increased ion extraction, possibly due to the retrograde motion of the plasma-vacuum edge. The laser ion source is utilized for carbon ion implantation of Ni(111), aiming for graphene synthesis. Ni(111) thin films are prepared with magnetron sputter coater on mica substrates at 500 °C with 400 nm thickness. Ni(111) thin films are analyzed with XRD and showed that the surface mostly contains single crystal Ni(111). Carbon at +5, -5, -10, and -15 keV of Ni (111) biasing with a series of dosages were implanted into Ni(111) films at room temperature. Carbon nanostructures such as amorphous carbon and diamond-like carbon were synthesized on Ni(111) substrates by the laser generated carbon ion implantation

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

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    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 ..

    Diamond based nanostructures for electronic applications

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    Research in the area of CVD diamond thin films has increased significantly during the last decades to the point where single crystal diamond is now commercially available. The remarkable properties of diamond including its extreme hardness, low coefficient of friction, chemical inertness, high thermal conductivity, transparency and semiconducting properties make it attractive for a number of applications, among which electronic devices is one of the key areas. A detailed knowledge of electrical properties of diamond films is therefore critical. This thesis describes (1) a Hall effect study of highly boron-doped (111) diamond films (2) a Hall effect and impedance spectroscopic study of boron δ-doped diamond structures and (3) the synthesis of carbon nanotubes on single crystal diamond. Systematic investigations have been carried out on single crystal, boron-doped (111) diamond films. The influence of ultra pure gases, doping concentration and temperature on carrier transport are discussed in detail. A comprehensive study on boron δ-doped diamond films is also performed; Hall effect and impedance spectroscopy are used to evaluate these films, providing valuable insight into the complex carrier transport mechanisms occurring in these structures. The influence of temperature on carrier mobility and the free carrier density are discussed. This is allied with valuable information gained from impedance spectroscopy, where the presence of multiple semicircular responses (conduction pathways), modelled using a RC parallel circuit, yields data which leads to a greater understanding on the influence of the interface between the boron δ-doped layer and the surrounding intrinsic diamond layers. These semicircular responses are thus attributed to different crystalline regions in these structures, namely the boron δ -doped layer and the interfacial regions surrounding δ-layer. The influence of this interface region on the structures overall conductivity is discussed. Finally the synthesis of carbon nanotubes (CNTs) on single crystal diamond is reported for the first time. Scanning electron microscopy combined with Raman spectroscopy is used to understand the influence of temperature and differing growth gas mixtures on the yield and crystallinity of these as-grown CNTs

    The processing of heteroepitaxial thin-film diamond for electronic applications

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    Electronic Properties and Applications of Nanodiamond

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    In recent years advances in the processing and purification of detonation nanodiamonds has renewed interest into their research, from the basic properties of detonation nanodiamonds to their applications in areas from electronics to biology. Using a colloid of mono dispersed detonation nanodiamonds it is possible to coat various substrate materials. This thesis reports on the suitability and enhancement of nanodiamond coatings for electronic applications. Atomic force microscopy is used to investigate the deposition of nanodiamond particles on substrates. The electrical characteristics of mono-dispersed nanodiamond layers are investigated using impedance spectroscopy, establishing that the layers have high quality dielectric characteristics. Hydrogen terminated CVD diamond is known to have a negative electron affinity (NEA), making it a suitable material for secondary electron emission. This thesis investigates using and optimising nanodiamond coatings on microchannel plates (MCPs) to increase the secondary electron yield of these devices, thereby improving the performance of image intensifiers. The as-received nanodiamond is covered with surface functional groups dependent on post detonation treatments for cleaning and deaggregation. Treatments have been designed which modify the surface groups for homogeneity, followed by an oxidation treatment to provide a platform for metallisation, notably caesium oxide which is known to give a stable and larger NEA surface thus further improving the secondary electron yield. Fourier transform infra-red spectroscopy and has been used to investigate the presence of functional groups. A comprehensive study of the secondary electron emission yield of nanodiamond coatings after various surface treatments is presented. The most effective treatment is found to be a low temperature chemical vapour deposition process which is compatible with the fragile MCP structure. SEM and Raman spectroscopy have been used to provide an insight into the changes of the material, which remains nanodiamond-like. These are the first such results from nanodiamond material
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