John O'Sullivan

Abstract:

Two novel chromium oxide arsenide materials have been synthesized, Sr2CrO2Cr2OAs2 (i.e., Sr2Cr3As2O3) and Sr2CrO3CrAs (i.e., Sr2Cr2AsO3), both of which contain chromium ions in two distinct layers. Sr2CrO2Cr2OAs2 was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO4As2 octahedra containing Cr2+ and distorted CrO2As4 octahedra containing Cr3+. In contrast, Sr2CrO3CrAs incorporates Cr3+ in CrO5 square-pyramidal coordination in [Sr2CrO3]+ layers and Cr2+ ions in CrAs4 tetrahedra in [CrAs]− layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr2CrO3CrAs the Cr2+ moments in the [CrAs]− layers exhibit long-range ordering, while the Cr3+ moments in the [Sr2CrO3]+ layers only exhibit short-range ordering. However, in Sr2CrO2Cr2OAs2, both the Cr2+ moments in the CrO4As2 environments and the Cr3+ moments in the CrO2As4 polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr3+ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr2CrO2Cr2OAs2 below the magnetic ordering transitions.

Abstract:

Effective temperature control is crucial in many studies of isolated biological tissues, with preparations often requiring specialized holding chambers. In these situations, the design flexibility and optimizations offered by a custom made temperature controller may be preferable over a commercial model. We present a versatile controller for heating and cooling applications, providing simple step-by-step instructions to mathematically model your specific system and optimize controller parameters. The apparatus uses analog components and linear stages to simplify circuit comprehension and customization, achieving fast transitions with small static errors and overshoots over a wide range of temperatures without readjustment. A fully featured rackable enclosure is complemented by two temperature probes based on the LMT70A linear microchip sensor (for the control loop and for bath monitoring). BNC outputs provide scaled probe signals for continuous temperature data acquisition. The maximum achievable power output of the controller is -23.5 W/+22.0 W (-4.7 V/+4.4 V, \ub15.0 A), sufficient to bring a well designed holder for standard 35 mm chambers from 23 \ub0C up to 37 \ub0C in ~1 min and down to 3 \ub0C in ~4 min. Any biologist with some technical prowess should be able to follow our instructions from modeling to assembly and calibration

Abstract:

Transparent conducting electrodes (TCEs) are a major source of efficiency loss in perovskite/silicon tandem solar cells. With the best-performing TCEs containing indium, they also compromise the sustainability of this promising renewable energy technology. Graphene, owing to its broadband transmittance and high mobility, offers great potential as an alternative TCE. However, its sheet resistance remains too high to be competitive against conventional TCE materials. Despite low sheet resistance being attainable via chemical doping, such techniques have poor stability and reduce transmittance, limiting their applicability in optoelectronic devices. This thesis explores how ion-charged dielectrics (ICDs) can be used to modulate the conductivity of graphene to achieve sufficiently low Rsheet to make it competitive with state-of-the-art TCOs in tandem solar cells.

In this thesis, a proof-of-concept for the ICD doping of graphene is first developed. The impact of a corona-charged dielectric on the properties of a graphene layer interfaced with it is characterised, demonstrating a graphene sheet resistance reduction of >75% to ~400 Ω/□. This is expanded upon by investigating the impact of migrated potassium ions in SiO2/Si on graphene sheet resistance. A record 117.9 Ω/□ sheet resistance is achieved – among the lowest reported in the literature for CVD monolayer graphene wet transferred to SiO2. Field-effect transistor characterisation indicates a charge concentration >6×10^12 /cm2 was generated on the graphene. The ability and stability of ionic charge in glass to dope graphene are also investigated, with sheet resistance <300 Ω/□ observed in vacuum after Na+ migration to the graphene/glass interface. This is contrasted with graphene encapsulation with corona-charged PMMA and AlOx. Although they can dope graphene, they have poor stability over time. Finally, the suitability of ICD-doped graphene for perovskite/silicon tandem cells is assessed via modelling. It is calculated that an efficiency gain of >1%abs may be achieved if ICD-doped graphene is used instead of state-of-the-art indium-based TCOs.