Search

The direction of research in solid state physics and technology has changed since the discovery of graphene. Now, a plethora of two-dimensional materials are being thoroughly investigated for their unique properties as well as for their implementation in next-generation optoelectronic devices. Of course, much effort is needed in order to reach the current level of modern electronics which is based on decades of research and development. For example, the level of miniaturisation modern technology requires can be achieved with atomically thin materials, driving Moore's Law forward. Conventional dielectrics exhibit high leakage currents when their dimensions are reduced to the nano-scale and the need for alternative materials compatible with two-dimensional electronics arises. However, the techniques that are being used for the growth and processing of conventional semiconducting materials are not always suitable with two-dimensional materials, which need special handling. These are some of the points that will be addressed in this PhD dissertation. Here, a new method for generating a fundamentally two-dimensional high-k dielectric which can be automatically incorporated in atomically thin optoelectronics devices is presented. The photo-oxidation of hafnium disulfide, HfS2, is a straight-forward, non-invasive process that can be used to oxidise pristine few-layered HfS₂, opening new paths for applications ranging from optoelectronics to photonics. The resulting dielectric, Hafnium dioxide, HfO₂, exhibits outstanding properties that exceed those of silicon dioxide, SiO₂ and its atomically thin nature makes it an ideal insulating layer for next-generation nano-electronics. Finally, the last part of this thesis is dedicated to a novel, CVD-grown, n-type monolayer of tungsten diselenide, WSe2. This is the first time negatively doped CVD-grown WSe₂ is reported, which opens the possibility of choosing the doping of the two-dimensional semiconductor before fabrication. For investigating and characterising this novel material, field-effect transistors are fabricated and characterised optoelectronically, shining light on the carriers' behaviour and the ability of the material in light-detection applications. Vacuum and ambient annealing of the WSe2 based devices highlights a possible way to control the doping level of the material, and thus the electrical behaviour of the devices.

The electronic structure of warm dense matter has been investigated using absorption spectroscopy by looking at shifts in the K-edge position of shocked compressed parylene-C and potassium chloride. The Vulcan laser facility was used to drive double sided shocks into the embedded chlorine samples creating a warm dense matter state with compressions greater than four times solid density and temperatures of around 10 eV. This resulted in red-shifts of the K-edge energy on the order of 10 eV. Also measured are differences in the edge energy with increasing density between the two types of chlorine samples, demonstrating how the embedded environment can effect the electronic structure of the plasma under warm dense conditions. The experimental results have been reproduced well by a reformulated version of the Stewart-Pyatt model that takes into consideration dynamical processes by including an additional relaxation energy exchange between the atom and its surrounding plasma when an electron is photo-ionised from the K-shell. In addition to this good agreement with the experimental results is also observed with simulations carried out using density functional theory.

Cavity enhanced spectroscopies (CES) are currently amongst the most sensitive spectroscopic techniques available for probing gas-phase samples, however their application to the liquid-phase has been more limited. Sensitive analysis of submicrolitre liquid samples is highly desirable, as miniaturisation allows for the reaction and analysis of scarce or expensive reagents, produces less waste, and can increase the speed of separations and reactions, whilst having a small footprint and high throughput. Absorption spectroscopy is a particularly desirable technique due to its universal, label-free nature, however its application to small volume liquid samples is hampered by the associated short absorption pathlengths, which limit sensitivity. CES improve sensitivity by trapping light within a confined region, increasing the effective pathlength through the sample. Three distinct types of optical cavity were constructed and evaluated for the purposes of making optical absorption measurements on liquid samples. The first incorporated a high optical quality flow cell into a "macrocavity" formed from two dielectric mirrors separated by 51.3 cm. Cavity losses were minimised by positioning the flow cell at Brewster's angle to the optical axis, and the setup was used to perform a single-wavelength cavity ringdown spectroscopy experiment to detect and quantify nitrite within aqueous samples. The detection limit was determined to be 8.83 nM nitrite in an illuminated volume of only 74.6 nL. Scattering and reflective losses from the flow cell surfaces were found to be the largest barrier to increased sensitivity, leading us to focus on the integration of cavity mirrors within a microfluidic flow system in the work that followed. In the second set of experiments, cavity enhanced absorption spectroscopy (CEAS) measurements were performed on Thymol Blue using custom-made microfluidic chips with integrated cavity mirrors. Unfortunately, due to the plane-parallel configuration of the mirrors and the corresponding difficulty in sustaining stable cavity modes, the results were underwhelming, with a maximum cavity enhancement factor (CEF) of only 2.68. At this point, attention was focussed toward a more well-defined cavity geometry: open-access plano-concave microcavities. The microcavities consist of an array of micron-scale concave mirrors opposed by a planar mirror, with a pathlength that is tunable to sub-nanometer precision using piezoelectric actuators. In contrast to the other experimental setups described, themicrocavities allow for optical measurements to be performed in which we monitor the change of wavelength and/or amplitude of a single well-defined cavity mode in response to a liquid sample introduced between the mirrors. In the first microcavity experiment, we used 10 μm diameter mirrors with cavity lengths from 2.238 μm to 10.318 μm to demonstrate refractive index sensing in glucose solutions with a limit of detection of 3.5 x 10-4 RIU. The total volume of detection in our setup was 54 fL. Thus, at the limit of detection, the setup can detect the change of refractive index that results from the introduction of 900 zeptomoles (500,000 molecules) of glucose into the device. The microcavity sensor was then adapted to enable broadband absorption measurements of methylene blue via CEAS. By recording data simultaneously from multiple cavities of differing lengths, absorption data is obtained at a number of wavelengths. Using 10 μm diameter mirrors with cavity pathlengths from 476 nm to 728 nm, a limit of detection, expressed as minimum detectable absorption per unit pathlength, of 1.71 cm-1 was achieved within a volume of 580 attolitres, corresponding to less than 2000 molecules within the mode volume of the cavity. Finally, a new prototype was developed with improved cavity finesse, a much more intense and stable light source, and improved flow design. Using a single plano-concave microcavity within the array with a cavity pathlength of 839.7 nm, and 4 μm radius of curvature mirror, absorption measurements were performed on Methylene Blue. Analysis of this data indicated a CEF of around 9270, and a limit of detection based on the measured signal-to-noise ratio of 0.0146 cm-1. This corresponds to a minimum detectable concentration of 104 nM Methylene Blue, which given the mode volume of 219 aL, suggests a theoretical minimum detectable number of molecules of 14.

This thesis reports work extending the scope of a recently developed gas sensing technique, multi-mode absorption spectroscopy (MUMAS). The ability of MUMAS to simultaneously detect multiple species from a mixture is demonstrated for the first time. The technique is subsequently extended to mid-infrared wavelengths, realising large gains in sensitivity. A solid-state, multi-mode laser has been developed to provide a high-performance comb source for use with MUMAS. This in-house constructed, diode-pumped, Er/Yb:glass laser operates on 10 longitudinal modes, separated by 18 GHz and centred close to 1565 nm. The extensive development and prototyping work leading to this final laser design is described. Multi-species detection with MUMAS is reported for the first time, thus demonstrating the ability of this technique to perform multi-gas sensing using a single laser and simple detection scheme. The previously described Er/Yb multi-mode laser was used to record MUMAS signals from a sample containing CO, C2H2, and N2O. The components of the mixture were detected simultaneously by identifying multiple transitions in each of the species. Temperature- and pressure-dependent modelled spectral fits to the data were used to determine the partial pressures of each species in the mixture with an uncertainty better than +/-2%. Multi-mode radiation has been successfully generated at 3.3 μm using quasi phase matched difference frequency generation (QPM-DFG). A mid-infrared laser comb was produced by optically mixing the near-infrared, multi-mode comb produced by the previously developed Er/Yb:glass laser with the single-mode output of a Nd:YAG laser operating at 1064 nm. This multi-frequency laser source was characterised to verify performance, and subsequently used to perform proof-of-principle MUMAS measurements on the strong transitions found in this spectral region. Spectra were recorded of NH3 and CH4 both individually and as components of a mixture. A minimum detection level for this system was determined to be 4.3 μbar m-1 for CH4, a sensitivity increase of 300 over similar measurements performed in the near-IR.

Cavity enhanced absorption spectroscopies have earned themselves a place as one of the methods of choice for sensitive absorption measurements on gas-phase samples, but their application to liquid samples has so far been more limited. Sensitive short pathlength analysis of liquid samples is required for online analysis of microfluidic samples, which are processed in channels with dimensions of tens to hundreds of micrometres. Microfluidics is important for a range of applications including drug discovery and environmental sensing. This thesis explores the application of cavity enhanced absorption spectroscopies to short pathlength (0.010 mm to 2 mm) analysis of sub-microlitre volumes of liquids. Three experimental set-ups have been been examined. Firstly, a single-wavelength cavity ringdown (CRD) spectrometer operating at 532 nm was assembled using two 99.8% reflectivity mirrors. High optical quality flow cells with short pathlengths ranging from 0.1 mm to 2 mm were inserted into this cavity at Brewster’s angle. The detection limit of the set-up with each inserted flow cell was established using a concentration series of aqueous potassium permanganate (KMnO₄) solutions. For the 1 mm flow cell, a detection limit of 29 nM KMnO₄ or 1.4 x 10⁻⁴ cm⁻¹ was established. Several different types of microfluidic devices were also inserted into the cavity, and it was found that the losses arising from the inserted chip were highly dependent on the method of chip manufacture. The CRD set-up with inserted 1 mm flow cell was applied to the detection of two important species, nitrite and iron(II), via analyte-specific colourimetric reactions. Detection limits of 1.9 nM nitrite and 3.8 nM iron(II) were established. The second experimental set-up utilised broadband, supercontinuum light generated in a 20 m length of nonlinear photonic crystal fibre. Broadband mirrors with around 99% reflectivity over the wavelength range from 400 to 800 nm were used to form the cavity, and a miniature spectrometer was used to wavelength-resolve the time-integrated cavity output. Flow cells and microfluidic chips were inserted into the cavity either at normal incidence or at Brewster’s angle. This set-up was employed for reaction analysis of an iron complexation reaction with bathophenanthroline, and for a model organic reaction, the Diels-Alder reaction between anthracene and 4-phenyl-1,2,4-triazoline-3,5-dione. The same broadband set-up was also used for pH measurements using bromocresol green indicator solution. Using dual-wavelength CRD spectroscopy, the pH sensitivity was established to be around a few milli pH units. Finally, an alternative type of cavity, formed from a loop of optical fibre has been investigated. A novel light-coupler was designed and fabricated in 365 μm core diameter multimode optical fibre. Sample designs employing both direct and evanescent wave absorption were investigated in small-core and large-core optical fibres, and the lowest detection limit of 0.11 cm⁻¹ was determined in direct absorption measurements, with a pathlength of 180 μm, using our novel light coupler in 365 μm core diameter optical fibre.

Mass diffusion coefficients of gas mixtures have been measured for more than 100 years. However, experimental data for the effective diffusion coefficient (Deff) of gas mixtures in the rarefied gas regimes at Knudsen numbers (Kn) above 0.1 are few and remain uncertain due to the increased effect of gas-wall collision behaviour. Due to growing demand in rarefied gas regime applications like microfluidic and vacuum systems, accurate Deff measurements are needed to validate available diffusion theories and empirical relations in the rarefied gas regimes. This thesis is the first to report the non-intrusive, in-situ, direct measurement of Deff of gas mixtures in the Kn range 0.1 < Kn < 6. The experimental investigation of Deff for gas mixtures in the transition gas regime where no direct measurements were available earlier has been made possible due to the implementation of the tunable diode laser absorption spectroscopy (TDLAS) technique using a hollow-core photonic crystal fiber (HCPCF) as the diffusion environment. First, different experiment methodologies used in the literature to measure the diffusion coefficient of gas mixtures were investigated. Information is synthesised to determine the limits of the current state of the art for measuring Deff in the transition gas regime, and reasons for the lack of Deff measurements at Kn above 0.1 were identified. As a result, the two-bulb (TB) diffusion configuration having the HCPCF combined with the TDLAS gas concentration measurement technique is proposed to measure the Deff of gas mixtures in the transition and free molecular gas regimes at Kn above 0.1. Second, this experiment methodology was implemented, and the Deff of binary and ternary gas mixtures containing CO2 as one of the gas species were measured in the transition gas regime. CO2 has strong absorption features near the wavelength of 2005 nm, corresponding to a wavelength range of one of the laboratory’s lasers that would propagate without significant attenuation through the available HCPCF. Selected rotational-vibrational transitions of CO2 near 2005 nm are probed using the TDLAS technique to monitor the change in the mole fraction of CO2 through the change in the path-integrated absorbance (∫Aexp) of CO2 absorption lines in real time as the diffusion progresses. The change in ∫Aexp as a function of time has been used to estimate the Deff of a gas mixture from the diffusion time constant. He and Ar were chosen as gas species in a gas mixture containing CO2 for simplicity and to avoid potential adhesion and surface reaction with the silicon dioxide (SiO2) wall surface of the HCPCF used in the study. Initially, the Deff of He-CO2 was measured closer to the continuum gas regime at Kn ≈ 0.01 and compared with literature to validate the Deff measurement using the TB-TDLAS methodology against more traditional measurements and empirical correlations. Then, the measurement was extended to the transition gas regime at 0.1 < Kn < 6 and the Deff of binary gas mixtures He-CO2, Ar-CO2, and ternary gas mixture He-Ar-CO2 were measured using the TB-TDLAS methodology. Third, continuing the in-situ and direct measurement of Deff, the advantage of species-specific diffusion measurement was demonstrated by reporting for the first time the simultaneous measurement of Deff of both gas species in a diffusing N2O-CO2 gas mixture in the transition gas regime. The measured Deff of binary and ternary gas mixtures in the transition gas regime is compared against the predicted Deff using the Bosanquet empirical relation. The comparison between the measured and predicted Deff of gas mixtures is discussed in detail. Finally, the measured Deff of He-CO2, Ar-CO2, and He-Ar-CO2 gas mixtures were validated against the direct simulation Monte Carlo (DSMC) calculations in the transition gas regime. The DSMC simulated Deff are closer to the measured Deff values if fully diffuse reflection of particles from the gas wall is assumed. Both the measured and DSMC simulated Deff demonstrate the viability of TB-TDLAS methodology as a convenient technique to obtain a non-intrusive, in-situ, direct measurement of Deff for gas mixtures in the transition gas regime. The research outcomes point out the possibility of further extending the TB-TDLAS experiment methodology to obtain the Deff of gas mixtures in the free molecular gas regime at Kn > 10. In addition, the TB-TDLAS methodology opens up the possibility of real-time monitoring of the change in the mole fraction of multiple gas species during the diffusion process, leading to the simultaneous measurement of Deff of multiple gas species involved in a gas mixture, applicable to all gas regimes from continuum to free molecular.

A new route to a well-defined block copolymer with alternating PEO-solubilising groups and fluorinated distyrylbenzene units was established. The Horner Wittig reaction was used as the polycondensation reaction. The non-fluorinated analogue of this block copolymer was prepared via the Wittig reaction. Both polymers were soluble in chloroform and free-standing films could be cast from solution. The position of the HOMO and LUMO energy levels of these two materials were determined by a combination of cyclic voltammetry, UV photoelectron spectroscopy and UV/Vis absorption spectroscopy. The presence of fluorine substituents on the distyrylbenzene unit had no influence on the HOMO-LUMO band-gap (3.0 eV). However, the position of these two energy levels relative to the vacuum level was shifted to higher energies (0.85 eV shift) in the case of the fluorinated block copolymer. The photoluminescence quantum efficiency of the fluorinated block copolymer was 17%, that of the non-fluorinated block copolymer was 34%. The former was used as the electron conducting layer in a light emitting diode with poly(p-phenylene vinylene) as the emissive layer. The latter was used as the emissive layer in light emitting diodes. Luminances over 2000 cd/m(^2) were observed for devices based on the non-fluorinated block copolymer using indium tin oxide as the anode and aluminium as the cathode. The luminescence efficiency of such devices was as high as 0.5 cd/A, corresponding to an internal quantum efficiency of 1.1%.Furthermore, an oligo(p-phenylene vinylene) was synthesised that contained two terminal fluorinated benzene rings and two central non-fluorinated benzene rings, all connected by vinylene bridges. This material aggregated in a 'brickwall' motif, where each molecule overlaps with two halves of molecules in the row above and below. The structure of this J aggregate is due to aryl-fluoroaryl-interactions and was demonstrated by X-ray crystal structure analysis.

A recently constructed novel analytical tabletop terahertz (THz) chemical sensor capable of detecting a wide range of gases with high sensitivity and specificity was characterized to assess its performance over a range of operational parameters. The sensor was designed with an objective of quantifying composition of exhaled human breath, where target concentrations span part per trillion (ppt) to part per billion (ppb) level of dilutions. The sensor utilizes terahertz rotational spectroscopy of sampled gases for quantification of dilutions. The sensor occupies a volume of ~ 2 ft3 and incorporates a coiled absorption cell, thermal desorption tubes, and all necessary electronic components necessary for autonomous operation. Coiled absorption cell minimizes the sensor footprint while maintaining a large path length for sensitive spectral measurements. Preconcentration aides the detection of compounds by removing the background gases which would negatively affect the absorption signal if present during spectral analysis. Spectral parameters of the sensor were studied to optimize its sensitivity. Efficiencies of preconcentration over a range of gas sampling parameters were determined by comparing concentrations measured by the sensor to concentrations of a reference gas mixture. The sensor was characterized in its ability to detect acetaldehyde, acetone, ethanol, isoprene, and methanol – all known breath analytes. These gases were chosen for their range of volatility and absorption strength. Minimum detectable sample concentrations are well suited for breath sampling making this sensor a valuable new tool for environmental sensing and biosensing.

This dissertation describes laser absorption spectroscopy methods developed for temperature and carbon oxide (CO and CO2) sensing in high-pressure, fuel-rich combustion conditions of hydrocarbon-fueled bipropellant rockets. The scope of the work includes fundamental studies of spectroscopic interactions at high gas density, development of unique laser tuning and signal processing methods, and application of prototype sensors to rocket combustion devices under investigation at the Air Force Research Laboratory in Edwards, CA. Infrared vibrational spectra of CO and CO2 were probed using tunable semi-conductor lasers to infer gas properties. Initial sensor design targeted the absorption spectra of CO near 4.98 μm, selected to minimize spectral interference with other combustion gas species at the extreme temperatures (> 3000 K) and pressures (> 50 atm) of a kerosene-fueled rocket combustion environment. Successful measurements were conducted up to 70 bar utilizing a scanned wavelength modulation spectroscopy technique, creating a new pressure-limit for quantitative in situ species sensing in a combustion device. At higher pressures (which were tested), collisional-broadening effects blended the targeted absorption transitions, causing differential absorption to diminish and reducing the signal-to-noise ratio of the measurements.To overcome the pressure-constraints, a more advanced laser absorption sensing strategy was developed, targeting the vibrational bandheads of CO near 2.3 μm and CO2 near 4.2 μm and exploiting the band narrowing effects of collisional line mixing to counter collisional broadening. Spectral line mixing—typically observed at high gas densities in which intermolecular collisions are sufficiently frequent and strong to cause a shift in energy level populations—corresponds to a transfer of absorption intensity from weak to strong absorption regions, inducing a narrowing of spectral features. This non-ideal phenomenon is more prominent in spectrally dense regions, such as bandheads. Targeting infrared bandheads to exploit line mixing, measurements of CO and CO2 concentration were demonstrated over a range of high pressures up to 105 bar in a single-element-injector RP-2/CH4-GOx rocket combustor. To make such measurements quantitative,spectroscopic models accounting line mixing effects have been developed utilizing a high-enthalpy shock tube; these models are then employed for interpretation of measured absorption signals for quantitative temperature and species sensing.

Database parameters for line spectra are collected from many studies and require independent verification for use in the design and measurements of laser absorption spectroscopy systems. While many lower temperature lines are well characterized, the higher temperature and pressure lines found in studies of combustion products rely on theoretical scaling. High-temperature calibration devices are therefore desired to allow experimental validation of parameters under test conditions. For this research, the design of a high-temperature, three-zone gas cell is proposed as a replacement for the current absorption cell system at The University of Tennessee Space Institute (UTSI). This new design, a prototype of which has been built and tested at The Technical University of Denmark, allows for measurements of test gas samples at uniform temperatures of at least 1300K. A computational fluid dynamics (CFD) model is shown for the current UTSI gas cell that has

Graphene is a layer of carbon atoms arranged hexagonally such as in the basal plane of graphite. Graphene has unusual properties due to this two-dimensional structure. The material is often deposited on copper and can be transferred to other substrates. The adsorption of adventitious functional groups and atomic diffusion at grain boundaries can modify both its interfaces. The graphene surface and its substrate-interface have been characterised using synchrotron light at energies spanning the carbon 1s absorption edge. This method selects carbon bonds via the resonant excitation of atomic electrons of carbon into molecular orbitals. Polarisation-dependent reflection spectroscopy has been advanced as a new characterisation method for substrate-supported graphene. This has been achieved through the determination of relevant refractive indices of graphene and carbonaceous adsorbates. The imaginary part of the indices was measured with absorption spectroscopy. The real part was derived using Kramers-Kronig Transformations. Theoretical models have shown that reflection spectra of graphene-based systems depend strongly on the reflectance difference between layers. This explains the sensitivity of reflection spectroscopy in distinguishing graphene from other carbonaceous layers. Absorption and reflection spectroscopies have been performed on graphene on copper along with complementary characterisation methods. Results confirm that the anisotropy of carbon bonds in graphene deteriorates when adventitious groups are adsorbed and can be recovered when these groups are removed by vacuum annealing. Immediately after annealing the graphene is in direct contact with the copper substrate, consistent with a measured increase in the carbon 1s binding energy. Density Functional Theory has informed experimental results and has shown that adsorbates affect the electronic structure of graphene. As the structural anisotropy deteriorates, carbon and oxygen intercalate at the substrate-interface and functional groups are adsorbed. Importantly, the structural integrity of graphene is preserved. Graphene transferred to other substrates typically has carbonaceous interface layers that are three times its own thickness. The graphene surface is covered with a layer of adventitious molecular groups such as carboxyl. The characterisation techniques pursued in this work and their outcomes will aid the design of future graphene-based systems in new nano- and biotechnologies.