Ultrasensitive Force Sensing with Nanospheres in an Optical Standing Wave Trap

This thesis details the experimental progress developing a state-of-the-art neutral force sensor using an optically levitated nano-sphere. Trapping 300 nm diameter fused silica spheres using a dual beam optical standing wave trap, we have experimentally reported a force sensitivity of 1.63 0:37aN=Hz1=2. Also we have measured the heating rate of the nano-spheres at high vacuum, and we have experimentally explored the nano-sphere dynamics to optimize the trap stability at pressures on the order of 10−6 mbar. At this pressure, with averaging times on the order of 105 s, we have demonstrated force sensing at 5.8 1.3 zN.The experimental merit of this ultra-sensitive neutral force sensor is to test for non-Newtonian gravity-like forces on the micron scale. In order to push our sensitivities even further toward this goal, we have developed a second optical standing wave trap system. Using two laser frequencies, 1596 nm and 1064 nm, incident on a single hemispherical optical cavity, we have successfully trapped nano-spheres inside the optical standing wave trap. In this thesis we outline the frequency stabilization system, the optical cavity, and experimental progress in overlapping both optical standing wave traps. We will outline further improvements that can and are being made to improve the system. Also included is an outline of our data analysis using Matlab to show nano-sphere displacement and force measurement

Carbon nanotubes and other highly curved surfaces for field emission and field-promoted ionisation

The thesis describes the development of various novel emitters for the production of gaseous ions from solutions of non-volatile, thermally labile samples for the purposes of mass spectrometry. Nano-electrospray emitters each containing two separated channels running throughout the length of the emitter were fabricated and evaluated. These emitters were made from “theta-shaped” borosilicate capillaries, employing a number of different coating procedures. Loading of different solutions into the channels demonstrated the possibility of studying solute interactions on ultrashort timescales. It is proposed that interactions took place in a shared Taylor cone. The formation of specific adducts from vancomycin and diacetyl-l-lysyld- alanyl-d-alanine was observed by mass spectrometry. From consideration of the extent of H/D exchange between vancomycin and deuterated vancomycin, it was concluded that the interaction times were of the order of 10-5 s. Underlying theoretical considerations, design and fabrication from carbon nanotubes (CNTs) of emitters for field desorption and field ionization ion sources are described and discussed. The emitters fabricated made use of arrays of vertically aligned multi-walled CNTs with in most cases an average length and radius of 15 μm and 35 nm respectively. Emitters using dense coverings of nanotubes and emitters with nanotubes selectively grown so that the height of each nanotube was twice its separation from its nearest neighbour were investigated. Characterisation of the CNTs by field electron-emission confirmed their effectiveness as field emitters. Fowler-Nordhein plots indicated fields of 6.14x109 +/- 0.72x109 V/m at a potential of 700 V. Field ionization of He, Ar, Xe, methane and acetone was achieved with these same CNTs; neither the inert gases nor methane have been field ionised with conventional activated-wire emitters. The fields generally accepted to be required for field ionisation of He and Ar are of the order of several 1010 V/m. To create emitters which would not need to be removed from vacuum between experiments, a means of injecting both liquids and gaseous samples directly to the bottom of the CNTs was devised. This involved drilling 20 μm diameter holes through the silicon substrate between intended sites of nanotube growth, but before actually growing the CNTs. It was discovered that the presence of the holes led to surface migration of the nickel catalysts initiating CNT growth. Experiments undertaken to achieve mass spectrometric measurements with the arrays of CNTs as emitters are described and discussed

Carbon Nanotubes and Tungsten Oxide Nanorods: Synthesis and Applications

Synthesis and applications of two types of one-dimensional nanomaterials, carbon nanotubes (CNTs) and tungsten oxide nanorods, are investigated in this dissertation. Multi-walled CNTs have been successfully synthesized using two types of chemical vapor deposition (CVD) methods: microwave plasma enhanced CVD and atmospheric pressure thermal CVD. CNTs and their synthesis processes are characterized with various analysis techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and optical emission spectroscopy. Ultra-thin and high quality multi-walled CNTs are discovered in CNT films produced by MPCVD, which exhibit good field emission performance that is found to be dependent on the synthesis conditions, like the growth time and CH4/H2 flow ratio. CNTs grown by thermal CVD have similar field emission performance. Based on silicon surface micromachining techniques and thermal CVD method, a self-aligned method has been developed to fabricate CNT based gated field emitter arrays (FEAs) which demonstrate low turn-on voltage and good emission current. Tungsten oxide nanorods have been synthesized on various tungsten substrates via thermal annealing in argon at atmospheric pressure. Nanorod growth mechanism is proposed based on thermal oxidation of tungsten in gas ambient with a very low partial pressure of oxygen as well as the self-catalytic effect on tungsten surface. The lattice structure and composition of the tungsten oxide nanorods are observed and analyzed using high resolution TEM, selected area electron diffraction (SAD), and energy dispersive X-ray spectroscopy (EDXS). The analysis results reveal that the lattice structure of the tungsten oxide nanorods is closest to that of the monoclinic WO3 crystal. Tungsten oxide nanorods have been successfully grown on tungsten tips for use in scanning tunneling microscope (STM) as probes which readily produce atomic resolution images on sample surface. Nanorod based FEAs are also successfully fabricated using similar techniques as those for fabricating CNTs based FEAs. Low turn-on voltage and low gate current are achieved

Development of carbon nanotubes with a diamond interlayer for field electron emission and heat transfer applications

Carbon Nanotubes (CNTs) have great potentials for Field Electron Emission (FEE) and Flow Boiling Heat Transfer (FBHT) applications. However, their weak adhesion on metallic substrates limits the development of CNTs in both applications. Diamond has high thermal conductivity and develops strong bonding with CNTs. The development of a diamond interlayer between CNTs and substrates is a feasible approach to address the adhesion problems. The purpose of this research was to develop a new CNT-based materials with a diamond interlayer for FEE and FBHT applications by focusing on four objectives: (1) enhancement of diamond thin film adhesion on a Cu substrate, (2) improvement of the CNT FEE stability, (3) reduction of the CNT FEE turn-on field, and (4) investigation of the FBHT performance of CNT based structures. The CNTs and diamond thin films in this thesis were prepared by Microwave Plasma enhanced Chemical Vapor Deposition (MPCVD) and Hot Filament enhanced Chemical Vapor Deposition (HFCVD). The structure and chemical states of the diamond films and CNTs were characterized by Scanning Electron Microscopy (SEM), cross-sectional Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), Raman spectroscopy, synchrotron based X-ray Absorption Spectroscopy (XAS). To deposit diamond thin films on a Cu substrate with sufficient adhesion strength, a sandblasting pretreatment and alloying with a tiny amount of Al were investigated. The adhesion of diamond thin films to substrates was evaluated by Vickers micro-hardness indentation. The FEE stability and turn-on field were measured by a Keithley 237 high voltage measuring unit. The FBHT property of the structures was tested repeatedly at different flow velocities to explore the dependence of heat transfer performance on certain parameters, including the flow patterns, Critical Heat Flux (CHF), and stability. The results show that sandblasting pretreatment increases the surface roughness and surface defect density, thereby increasing diamond nucleation density and adhesion to the Cu substrate. Al alloying appears to inhibit the formation of graphite at the interface between diamond and the Cu substrate, which improves the chemical bonding between diamond and the Cu substrate and increases the adhesion strength between them. The FEE testing results show that ultra-high FEE stability (more than 5000 minutes) was achieved for the CNTs with a diamond interlayer. This is attributed to the good contact at the diamond-CNT and diamond-substrate interfaces. The main factors that affect the CNT FEE turn-on field were also studied. By optimizing the structure, an FEE turn-on field of 5.1 V/μm was achieved and an emission barrier model for CNTs with a diamond interlayer on Cu substrate was used to explain the results. FBHT testing was done on CNTs with different structures and the results show that high heat transfer efficiency can be achieved on CNTs with a diamond interlayer at low mass fluxes

Development of carbon nanostructures from non-conventional resources

Carbon nanostructures (CNSs) perpetuate the scientific interest over decades due to their remarkable properties and emerging technological applications. The development of sustainable technologies for the synthesis of CNSs from natural resources grabbed immense research attention aiming to implement these high-end materials in wide range of nano electronic devices through safe and environmentally friendly routes. Even though a number of top down and bottom up approaches have been developed for the production of CNSs, most of them either aided by catalysts or involved solvent assisted multi-step process that considerably increase the cost of production and hinders the realization of low cost CNSs based commercial devices. In addition, vast majority of these techniques use high pure petroleum derived hydrocarbon gas precursors that are non-renewable and expensive. Hence, it is imperative to develop scalable techniques that can derive high quality CNSs directly on arbitrary substrates from naturally derived carbon feed stocks. This work aims to develop an environmentally benign plasma enhanced chemical vapor deposition technique for fabricating CNSs from Citrus sinensis essential oil, a bio renewable precursor, and explored the potential of these nanostructures for gas sensing application. C. sinensis essential oil, obtained through cold extraction of orange peels is a rich source of non-synthetic hydrocarbon compounds principally limonene. Inherently volatile in nature, C. sinensis essential oil can serve as an ideal candidate material compatible to plasma enhanced chemical vapor deposition. This thesis investigated the fabrication of vertically-oriented graphene nanostructures from C.sinensis essential oil through radio frequency plasma enhanced chemical vapor deposition process, the fundamental properties, extend to which the process parameters influenced the structure and morphological features, and the suitability of the developed vertical graphene arrays for gas sensing applications. Special attention is paid to probe deep into the morphological evolution with the help of a set of advanced analytical characterization methods and multi-parameter model simulations. In the first phase, C.sinensis vapors were subjected to low RF power glow discharge that resulted in the formation of plasma polymer thin films, and the material properties were studied as a function of input RF energy. The fundamental properties of plasma polymer thin films fabricated at different RF power level (10−75 W) were characterized with variable angle spectroscopic ellipsometry, UV-visible spectroscopy, Fourier transform infrared spectroscopy X-ray photoelectron spectroscopy and atomic force microscopy. Optical characterization showed that independent of deposition power films exhibited good transparency (~90 %) in the visible region and a refractive index of 1.55 at 500nm. The optical band gap measured around 3.60 eV and falls within the insulating region. The atomic force microscopic (AFM) images revealed that the surface is pinhole-free and smooth at nanoscale, with average surface roughness dependent on the deposition power. Film hardness increased from 0.50 GPa to 0.78 GPa as applied power increased from 10 to 75 W. In the second phase, experiments were modified by redesigning the experimental set up in order to eliminate hydrogen from the deposits leaving only crystalline carbon. The RF power deliberately kept high, substrate temperature was raised and hydrogen gas fed into the reactor in controlled manner. A sequence of experiments were carried out by systematically changing the process parameters such as in put RF power (300-500W), hydrogen flow rate (10-50 sccm) and deposition duration (2-8 min) and analysed the structural and morphological evolution of the resulted vertical graphene nanostructure. The structure-property correlation of vertical graphene arrays with the plasma process parameters was performed. The Raman spectra ascertained the formation of less defected multilayered graphene nanostructures and scanning electron microscopic images provided the primary evidences of morphological evolution. The potential of the novel analytical techniques such as Hough transformations, fractal dimension distributions and Minkowski connectivity for the analysis of graphene array morphology was then successfully demonstrated. Worth noting that, these advanced techniques displayed significant changes and revealed the complex morphological transformation of C. sinensis derived vertical graphene subjected to change in process conditions. Precisely, vertical graphene nanowalls obtained at 300 and 500W presented a narrow height distribution profile but much wider array formed at 400 W. Fourier and Hough transformation spectra showed a prominent change with an increase in power, thus highlighted change in the morphology with the density of nanoflakes. Similarly, 2D FFT transform spectra of vertical graphene samples also presented notable changes with increasing hydrogen flux. The most narrow height distributions, well-shaped Hough transformation spectra and distribution of fractal dimensions obtained for structures formed at 20 and 50 sccm of hydrogen flow rate. In addition to this, the principal characteristics of thus fabricated vertical graphene such as flake length (Lvg) and flake half width (Wvg) are theoretically modelled by an ad hoc model based on a large number of interaction elemental processes and correlated with the experimental results. The combination of the experimental and simulation results ensured important insights and deeper understanding of the processes that govern formation of the vertical graphene morphology.Vertical graphene nanostructures having superior structural and morphological properties were successfully fabricated at an input RF energy of 500W, hydrogen flow rate of 30 sccm and deposition duration of 6 minutes. The third phase presented an in-depth study of the properties of C.sinensis oil derived graphene over a set of conducting (copper and nickel) and insulating substrates (silicon and quartz). The SEM images of thus fabricated graphene patterns showed the unique feature of vertically interconnected and non-agglomerated carbon nanowall structures having maze-like and petal-like networks. Moreover, the normalized height distribution function and 2-D FFT spectra analysis ascertained that vertical graphene formed on silicon substrates displayed the most uniform distribution. X-ray photoelectron spectroscopy spotted only the presence of carbon and the transmission electron microscopic studies revealed the formation of unique onion-like closed loops. The 3-D nanoporous structure of C.sinensis oil derived graphene showed high hydrophobicity and measured a water contact angle of 129°. The surface energy studies were performed using Neumann model, Owens-Wendt-Kaelble approach and van Oss- Chaudhury-Good relation and estimated within the range 35‒41 mJ/m². Finally, plasma reformed vertical graphene from C. sinensis was integrated into a sensor device prototype to evaluate the performance in gas sensing. The chemiresistive type sensor exhibited sensing activity towards acetone. In summary, this thesis has identified a viable renewable resource and successfully developed a process that transform them into vertical graphene nanostructures. Furthermore, the fabricated graphene was integrated to real world devices and evaluated the performance. The outcomes of this investigation add knowledge base to the state-of-the-art of green chemistry approach for the synthesis of vertical graphene carbon nanostructures

An Experimental and Computational Investigation into Laser-Based Synthesis and Spectrochemical Characterizations of Metal/intermetallic Nanoparticles with Engineered Interfacial Functionalities

Nanomaterials have, over the years, generated tremendous interests of scientists and engineers from nearly all disciplines. This interest has been due to a large number of desired physico-chemical properties such as magneto-optic properties, mechanical strength, melting points, charge transport behavior, and surface reactivity exhibiting unique size-dependent characteristics at the nanoscale. The unique interfacial properties are widely believed to be a result of high ratio of surface to bulk atoms as well as, bridging states in which nanoparticles exist between atomic and bulk materials. Thus, in the world of material processing and engineering, recent years have seen a surge in the use of wide classes of nanostructured materials as novel energetic, catalytic, semiconductor, and biomedical materials with engineered functionalities that find use in industrial, technological and defense applications. Therefore, it becomes imperative to develop fundamental understanding on the manufacturing and characterization routes that can allow the systematic tuning of the interfacial-property characteristics of advanced nanomaterials by tailoring their sizes and architectures. The current PhD thesis aims to address this grand-challenge engineering problem by investigating early-stage formations theoretically, synthesis and novel spectrochemical characterizations of advanced metal/intermetallic and composite nanoparticles (NPs) with engineered surface properties. Specifically, the thesis is categorized into two broad sections, namely laser-based synthesis studies and laser-based spectroscopic characterizations of NPs. The synthesis section presents theoretical investigations into the inception stage of NP formations, namely nucleation via numerical simulations. Briefly, this section aims to reveal the processing-structure-property relations of metal NPs synthesized via gas phase routes in an effort to relate the processing parameters to the size and morphology of the NPs, which in turn, dictates their interfacial energetic and catalytic behaviors. Then, using the obtained fundamental understandings a laser-based synthesis technique is presented for generating novel energetic metallic nanocomposites. The size, morphology and energetic activities of these materials are analyzed and tuned to improve the energetic properties. Finally, the laser spectroscopic characterization section focuses on experimental investigations by introducing laser induced breakdown spectroscopy (LIBS) as a relatively non-destructive and robust spectrochemical technique for the structural and chemical composition characterizations of composite NPs in a facile, yet effective manner

STUDY OF FIELD EMISSION AND IMPACT RESISTANCE CHARACTERISTICS OF FLUORINATED CARBON NANOTUBE COMPOSITES

Ph.DDOCTOR OF PHILOSOPH

Carbon nanotubes and other highly curved surfaces for field emission and field-promoted ionisation

The thesis describes the development of various novel emitters for the production of gaseous ions from solutions of non-volatile, thermally labile samples for the purposes of mass spectrometry. Nano-electrospray emitters each containing two separated channels running throughout the length of the emitter were fabricated and evaluated. These emitters were made from “theta-shaped” borosilicate capillaries, employing a number of different coating procedures. Loading of different solutions into the channels demonstrated the possibility of studying solute interactions on ultrashort timescales. It is proposed that interactions took place in a shared Taylor cone. The formation of specific adducts from vancomycin and diacetyl-l-lysyld- alanyl-d-alanine was observed by mass spectrometry. From consideration of the extent of H/D exchange between vancomycin and deuterated vancomycin, it was concluded that the interaction times were of the order of 10-5 s. Underlying theoretical considerations, design and fabrication from carbon nanotubes (CNTs) of emitters for field desorption and field ionization ion sources are described and discussed. The emitters fabricated made use of arrays of vertically aligned multi-walled CNTs with in most cases an average length and radius of 15 μm and 35 nm respectively. Emitters using dense coverings of nanotubes and emitters with nanotubes selectively grown so that the height of each nanotube was twice its separation from its nearest neighbour were investigated. Characterisation of the CNTs by field electron-emission confirmed their effectiveness as field emitters. Fowler-Nordhein plots indicated fields of 6.14x109 +/- 0.72x109 V/m at a potential of 700 V. Field ionization of He, Ar, Xe, methane and acetone was achieved with these same CNTs; neither the inert gases nor methane have been field ionised with conventional activated-wire emitters. The fields generally accepted to be required for field ionisation of He and Ar are of the order of several 1010 V/m. To create emitters which would not need to be removed from vacuum between experiments, a means of injecting both liquids and gaseous samples directly to the bottom of the CNTs was devised. This involved drilling 20 μm diameter holes through the silicon substrate between intended sites of nanotube growth, but before actually growing the CNTs. It was discovered that the presence of the holes led to surface migration of the nickel catalysts initiating CNT growth. Experiments undertaken to achieve mass spectrometric measurements with the arrays of CNTs as emitters are described and discussed.EThOS - Electronic Theses Online ServiceEngineering and Physical Sciences Research Council (Great Britain) (EPSRC)Bruker Daltonics (BD)GBUnited Kingdo

GRAPHITE OXIDATION AND DAMAGE UNDER IRRADIATION AT HIGH TEMPERATURES IN AN IMPURE HELIUM ENVIRONMENT

The High Temperature Gas-Cooled Reactor (HTGR) is a Generation IV reactor concept that uses a graphite-moderated nuclear reactor with a once-through uranium fuel cycle. In order to investigate the mechanism for corrosion of graphite in HTGRs, the graphite was placed in a similar environment in order to evaluate its resistance to corrosion and oxidation. While the effects of radiation on graphite have been studied in the past, the properties of graphite are largely dependent on the coke used in manufacturing the graphite. There are no longer any of the previously studied graphite types available for use in the HTGR. There are various types of graphite being considered for different uses in the HTGR and all of these graphite types need to be analyzed to determine how radiation will affect them. Extensive characterization of samples of five different types of graphite was conducted. The irradiated samples were analyzed with electron paramagnetic resonance spectroscopy, Raman spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy and gas chromatography. The results prove a knowledge base for considering the graphite types best suited for use in HTGRs. In my dissertation work graphite samples were gamma irradiated and also irradiated in a mixed field, in order to study the effects of neutron as well as gamma irradiation. Thermal effects on the graphite were also investigated by irradiating the samples at room temperature and at 1000 °C. From the analysi of the samples in this study there is no evidence of substantial damage to the grades of graphite analyzed. This is significant in approving the use of these graphites in nuclear reactors. Should significant damage had occurred to the samples, the use of these grades of graphite would need to be reconsidered. This information can be used to further characterize other grades of nuclear graphite as they become available

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