Physics - Doctoral Theses

Permanent URI for this collection


Recent Submissions

Now showing 1 - 5 of 101
  • Item
    Upconverting nanoparticles: pushing theory and technology towards biomedical applications
    (University College Cork, 2023) Souza Matias, Jean; Andersson-Engels, Stefan; Melgar Villeda, Silvia; Science Foundation Ireland
    Biophotonics faces a significant challenge in developing non-invasive imaging and interrogation techniques with high spatial resolution and penetration depth for precise diagnosis and treatment. These techniques rely on non-ionising radiation and non-toxic contrast agents possessing excellent photo and chemical stabilities. Among the various non-ionising wavelengths, the near-infrared (NIR) optical window offers the highest tissue penetration due to its minimal scattering and absorption in living tissues. Upconverting nanoparticles (UCNPs) possess the desirable properties of absorbing and emitting NIR light, making them ideal contrast agents for biomedical applications. UCNPs have shown promise in deep tissue imaging, optogenetics, photodynamic therapy, temperature sensing, drug delivery, and super-resolution microscopy. However, the efficiency of upconversion (UC) in UCNPs, as quantified by the quantum yield (QY), remains a significant challenge, particularly at low excitation power densities (PDs) where the non-linearity of upconversion luminescence (UCL) dominates. Furthermore, the lack of commercially available devices and standardised protocols that account for the crucial parameters affecting PD-dependent QY further complicates accurate characterisation of these materials. To address these issues, this thesis presents the design and construction of a comprehensive, broad-band, multi-variable QY characterisation system. This opto-electronically engineered setup enables simultaneous measurement of absorption and luminescence at two selected wavelengths. In addition, the fully automated system incorporates capabilities for characterising excitation beam profiles, scattering, and the emission spectrum of luminescent compounds in aqueous solutions. Accurate characterisation of the excitation beam profile is of particular importance due to its influence on the PD-dependent QY behaviour of UCNPs. Varying beam profiles lead to distinct QY values, necessitating beam profile compensation to derive an intrinsic QY property of the material independent of measurement configuration. However, achieving this compensation requires a comprehensive understanding of the mechanisms governing each UC emission wavelength, which has not yet been extensively studied across a wide range of PDs. Consequently, this thesis also includes a detailed theoretical study based on the rates of populating and depopulating the electronic energy states involved in UC processes. The investigation successfully quantifies the transition points at which the UCL transitions from non-linear behaviour of different orders to linearity. The theoretical model is validated using experimental UCL data acquired from two distinct UCNP compounds at various wavelengths. These combined theoretical and experimental studies are of utmost importance for the accurate characterisation and engineering of optimal UCNPs, representing a crucial advancement in the development of high-resolution, deep-penetration biomedical techniques and devices.
  • Item
    Quantifying greenhouse gas and air quality pollutant emissions using airborne measurements
    (University College Cork, 2023) Peischl, Jeffrey W.; Ruth, Albert A.; Wenger, John; Brown, Steven S.; National Oceanic and Atmospheric Administration; Hendrix College
    The composition of the atmosphere affects the health and well-being of people around the world. Air quality is a leading risk factor for mortality in the world today, and climate change may have a similar impact in the future. Knowledge of the magnitude of trace gas emissions to the atmosphere is necessary for policy makers who wish to most effectively reduce emissions that lead to harmful air quality or climate change. This knowledge typically starts with an inventory, where activity data are multiplied by emission factors to arrive at total emissions. Inventories are often used by policy makers to determine what emissions are and how they may be effectively reduced. Inventories are also used by scientists to model the effects of greenhouse gases on climate and the effects of ozone or particulate matter production on air quality. Scientific advancement in these areas in turn informs policy. Therefore, verifying inventories is necessary so that policy makers can make the most informed decisions to mitigate the negative atmospheric impacts of human activities. Here, I review multiple techniques used to verify inventories with measurements from trace gas analyzers installed aboard aircraft. The techniques are: (1) comparing an inventory emission ratio of two gases using atmospheric enhancement ratios; (2) comparing spatially broad airborne measurements to a local, in situ study using an atmospheric mixing model; (3) quantifying relative emissions using atmospheric enhancement ratios; and (4) directly quantifying emissions using airborne data. These techniques have been used to verify the accuracy of inventories and to reveal where improvements to inventories can be made.
  • Item
    Theory of carrier transport in III-Nitride based heterostructures
    (University College Cork, 2023-05-01) O'Donovan, Michael; Schulz, Stefan; O'Reilly, Eoin P.; Science Foundation Ireland; Sustainable Energy Authority of Ireland
    Wurtzite III-nitride materials and their alloys have attracted significant interest for solid state lighting applications. This is due to the direct band gaps of InN, GaN, and AlN crystals, which span a wide range of emission wavelengths. Due to the importance of these systems, the goal of optimising device performance has been an extremely active field of research. An important aspect of this is the development of improved modeling techniques. More recently, an emphasis has been put on understanding the impact the disordered alloy microstructure has on the electronic structure, however models focusing on transport properties are less mature. This is in part due to the challenges of connecting a random alloy description of the underlying microstructure with transport models. This thesis addresses this difficult problem by developing and utilizing different simulation frameworks, focusing on transport properties of (In,Ga)N/GaN quantum well systems. More specifically, the non-equilibrium Green's function (NEGF) formalism has been employed to study ballistic transport in a fully quantum mechanical setting. This builds on a tight-binding description of the electronic structure which ensures an atomistic description of the alloy is achieved. Our results indicate that while the alloy microstructure is of secondary importance for electrons, the transmission of holes is strongly perturbed by the presence of disorder. This is attributed to the breakdown of the translational symmetry of the system, which opens up new channels not present when fluctuations in local alloy content are neglected (using a virtual crystal approximation). Moreover, we have developed a new semi-classical multi-scale drift-diffusion model. This allows simulation of full devices due to a reduced computational demand compared to the NEGF formalism, while still keeping a microscopic resolution and accounting for important quantum corrections. The starting point is again the tight-binding model, which is used as a foundation to describe the alloy microstructure: A 3-dimensional energy landscape is extracted which includes an atomistic description of alloy fluctuations, local strain, and local polarization. This can be used as a confining potential for electrons and holes, and quantum corrections can be included in a numerically efficient manner via the recently developed localization landscape theory. This landscape, including or excluding quantum corrections, is used to study both uni-polar electron and hole transport. Our results show that, when quantum corrections are accounted for, the virtual crystal approximation is again a good approximation for electron transport, whereas hole transport is reduced due to carrier localization effects in the quantum well region. Finally this framework is extended to a p-i-n junction, where carrier (and thus recombination) distribution across a multi-quantum well system is studied. This system allows for a comparison between our in-house model and a commercial software package. Without including disorder in the alloy microstructure both schemes fail to reproduce literature experimental results. However, the situation changes when the random alloy microstructure is accounted for using our newly developed approach: The predicted behaviour is consistent with literature experimental results, without changing any other simulation parameters. These results highlight the importance of the treatment of the alloy microstructure in simulations, and indicate that our developed framework is an ideal starting point for modeling III-N systems to understand fundamental properties and guide device design.
  • Item
    Micro-transfer printing of micro-structured, ultra-thin light-emitting devices
    (University College Cork, 2023-03) Shaban, Zeinab; Corbett, Brian; Parbrook, Peter James; Science Foundation Ireland
    3D integration of optoelectronic devices is a crucial future technology, which can be applied in the areas of photonic integrated circuits, flexible displays, communication and more. Among the various technologies, micro-transfer printing has emerged as a precise and cost-effective way to assemble devices for 3D integration. To enable this technology, devices must be released from their native substrates, which open up a lot of possibilities. It can achieve integration with flexible or heat-conductive backplanes, as well as heterogeneous integration of multiple materials on a common platform, resulting in miniaturised chips. Also one can benefit from reclaiming and reusing the original substrates to reduce the production cost significantly. On the other hand, GaN devices exhibit unique optical properties in optoelectronics compared to other semiconductors, and GaN-based LEDs have established themselves in a variety of applications, due to their low power consumption, long lifetime, short response time, and high brightness. This thesis has focused on releasing high performance GaN LEDs and addressing their associated issue for micro-transfer printing. The first part of this work is focused on releasing and transfer-printing of GaN LEDs grown on Si substrate. There are several factors that limit the performance and manufacturing of GaN LEDs on Si. One issue is related to the deformation of the released coupons due to their high inbuilt strain, which could result in transfer-printing failures as well as challenges during the post-print integration process. To address this issue, COMSOL software was used to study the stress effect on the devices. Experimentally, the intrinsic deformation of the released LEDs was compensated by using compressed SiNx layers that resulted in flat devices after release. Another issue is related to the low light extraction for GaN LEDs on Si. To solve this problem, the underside of the released LEDs is roughened during the coupon preparation process prior to transfer printing. Furthermore, using the unique properties of transfer printing, the roughened LEDs are printed inside a fabricated reflective trench with 10 μm depth to direct the light to the surface normal. Results showed that roughening along with the reflective trench increased the collected power by a factor of ∼ 7 compared with LEDs on the original substrate. A second part of this study examines the release of GaN-based structures from substrates (i.e. sapphire or bulk GaN) by photoelectrochemical (PEC) etching when pure chemical etching is not possible. A sacrificial layer which can obtain smooth etch surfaces and uniform etching with high selectivity is needed. Also, from the perspective of transfer printing, thick rather than thin sacrificial layers are preferred to facilitate the releasing and picking process. In this work, 300 nm-thick releasing layers comprising of InGaN/AlInN stacks are proposed for PEC etching. The presence of two-dimensional hole gas at the interface of InGaN/AlInN due to the strong polarization field are indicated by modelling and capacitance-voltage measurement. This resulted in a smoother surface with a three times higher etch rate compared to the conventional InGaN/GaN superlattice structures used for PEC etching. Moreover, various electrolytes and post-PEC treatment were studied to improve the surface smoothness. Further work should be done to determine the impact of the adhesion layer in transfer printing on heat generation and device performance. Using the optimized sacrificial layer to release other structures like lasers should also be investigated.
  • Item
    Enhanced shortcuts to adiabaticity
    (University College Cork, 2022) Whitty, Chris; Ruschhaupt, Andreas; Irish Research Council
    Dynamical control of quantum systems is a fundamental requirement for designing and engineering quantum technologies. Adiabatic control methods are used extensively to control quantum systems in many settings. However, adiabatic control methods require long operation times. To address this issue, a collection of techniques called “shortcuts to adiabaticity" (STA) have been developed. STA have been applied in many settings, and they can offer significant improvement over adiabatic schemes. However, a major limitation of STA is that fully analytic STA schemes are known only for several specific families of quantum systems. Motivated to overcome this restriction, in this thesis we derive an analytic method called “enhanced shortcuts to adiabaticity" (eSTA) that extends STA techniques to systems that do not admit STA methods exactly. We first derive the eSTA formalism and demonstrate its utility in designing control schemes for several practical quantum control settings. We then investigate the robustness of eSTA against several types of systematic error and environmental noise, using the setting of neutral atom lattice transport. We also derive an alternative eSTA technique that naturally includes higher order terms, at the expense of further calculation. Both the alternative and original eSTA schemes are applied to fast anharmonic trap expansion. Finally, transport of two ions with Coulomb interaction in an anharmonic trap is considered and eSTA is shown to be robust against the effect of amplitude noise in this setting.