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Abstract: The adsorption of peptides on metal oxides is an area of significant interest, both fundamentally and in a number of technologically important areas. These range from the integration of biomaterials in the body, to denaturation of protein therapeutics and the use of biomolecules and bioinspired materials in synthesis and stabilization of novel nanomaterials. Here we present a study of the tripeptide arginylglycylaspartic acid (RGD) on the surfaces of vacuum-prepared single crystalline TiO2(110), pyrocatechol-capped TiO2(110), and model SLA and SLActive dental implant samples. X-ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy show that the RGD adsorption mode on the single crystal is consistent with bonding through the deprotonated carboxylate groups of the peptide to surface Ti atoms of the substrate. Despite the increased hydrophobicity of the pyrocatechol-capped TiO2(110) surface RGD adsorption from solution increases following this surface treatment. RGD adsorption on SLA and SLActive surfaces shows that the SLActive surface has a greater uptake of RGD. The RGD uptake on the pyrocatechol capped single crystal and the model implant surfaces suggest that the ease with which surface contaminant hydrocarbons are removed from the surface has a greater influence on peptide adsorption than hydrophobicity/hydrophilicity of the surface.

Open Access

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Abstract: The realisation of photonic devices for different energy ranges demands materials with different bandgaps, sometimes even within the same device. The optimal solution in terms of integration, device performance and device economics would be a simple material system with widely tunable bandgap and compatible with the mainstream silicon technology. Here, we show that gallium arsenide nanowires grown epitaxially on silicon substrates exhibit a sizeable reduction of their bandgap by up to 40% when overgrown with lattice-mismatched indium gallium arsenide or indium aluminium arsenide shells. Specifically, we demonstrate that the gallium arsenide core sustains unusually large tensile strain with hydrostatic character and its magnitude can be engineered via the composition and the thickness of the shell. The resulted bandgap reduction renders gallium arsenide nanowires suitable for photonic devices across the near-infrared range, including telecom photonics at 1.3 and potentially 1.55 μm, with the additional possibility of monolithic integration in silicon-CMOS chips.

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Abstract: Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties, and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced Voc but increased Jsc and FF, and therefore the ultimate influence to efficiency depends on the compensation of Voc loss and gains of Jsc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic bandgaps and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, with the conversion of ITIC to IT-4F reduces structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger π−π stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection towards anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encourage the intermixing of donor/acceptor for efficient charge generation.

Open Access

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Abstract: To date optoelectronic devices have been dominated by inorganic semiconductors such as silicon and gallium nitride. However organic semiconductors have many potential advantages and have shown significant research progress in both organic light-emitting diodes and organic photovoltaics. Tightly-bound excitons form intermediate states in the operation of both organic light-emitting diodes and organic photovoltaics. Many properties of an exciton are determined by its spin state, which can either be a spin-zero singlet or a spin-one triplet. In both organic light-emitting diodes and organic photovoltaics triplet excitons can be a source of loss due to poor radiative coupling to the ground state and energy loss on formation from the singlet state. In this thesis, methods of avoiding loss pathways through triplet states are investigated in both organic light-emitting diodes and organic photovoltaics. Carbene-metal-amides are a new class of emitter material for organic light-emitting diodes. High-performance devices are reported with maximum external quantum efficiency of 27%, which indicates effective triplet utilisation. Emission colour can be selected through both molecular design and host environment allowing the demonstration of blue-emitting devices with 19% external quantum efficiency. Host-free devices have been fabricated with record external quantum efficiency of 23%, enabled by a resistance to concentration-dependent luminescence quenching. Electron spin resonance has been used to investigate the mechanism of intersystem crossing between singlets and triplets in the important organic light-emitting diode material, 4CzIPN. It is demonstrated that the spin-vibronic model of intersystem crossing breaks down in device-relevant solid films. Evidence is instead found for back transfer from a spin-correlated radical pair state being the dominant mode of intersystem crossing. With increased film concentration the signal becomes dominated by free polarons, suggesting that charge separation may be a mechanism of luminescence quenching at high concentration. In many organic photovoltaic material systems triplets are low-lying states from which charge separation is not energetically favoured. In this thesis, attempts are made to negate this triplet loss pathway by the use of low singlet-triplet energy gap materials to ensure that the charge-transfer state can be endothermically accessed from all molecular exciton states.

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Abstract: Uniform, periodic and ordered iron oxide nanopatterns can be generated by selective metal ion inclusion into microphase separated polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) thin films. After solvent mediated metal ion inclusion into the PEO block, an ultraviolet-Ozone (UVO) treatment was used to remove the polymer and oxidize the metallic ions to their oxides. This paper provides an in-depth study of the UVO processing steps as a function of exposure time. Surface wettability, topography, morphology, compositional and interfacial changes were analysed by contact angle measurement, microscopic and spectroscopic techniques through the UVO treatment. It was found that the UVO treatment initially cross-links the polymer network followed by oxidation and removal of the polymer simultaneously. It was also found that if short UVO exposure times are used, a post calcination treatment can be used to generate similar patterns. The iron oxide nanopatterns created due to strong coordination bond between metallic ions and free electron pairs of O atoms in the PEO and these interactions are responsible for the final pattern mimicking the original self-assembled BCP morphology. The film thicknesses, surface roughness, the size/shape of the iron oxides and patterns, the amount of residual polymers were also investigated regarding the UVO exposure time.