Manifestations of Aging in Virtual Reality Implementation of Rod and Frame Test

Senior adults’ reliance on the visual frame of reference for spatial orientation is a manifestation of an age-related shift in cognitive style from field independence to field dependence. We implemented a virtual reality rod and frame test (VR-RFT) to assess visual field dependence (VFD) in n=39 young adults (20-30 years old) and n=43 seniors (60 years old and above). The subjects were asked to determine subjective visual vertical (SVV) for 19 angles of frame tilt (running from -45 degrees to 45 degrees in steps of 5 degrees). The strong VFD of seniors was manifested not only by the increased error in the determination of SVV (SVVE) but also in its distribution. For small and large frame tilt angles, seniors’ SVVE skewness and kurtosis were greater than those of young adults. The SVVE median dependence on frame tilt may be accounted for with a phenomenological model whose two parameters describe the strengths of primary (P) and secondary (S) visual attractors which subjects use to infer SVV: the edges of the frame and its imaginary diagonals. For young adults, these parameters were: PY=14.91 and SY=12.51. For seniors, we observed an over 50% increase in the strength of the primary attractor PS=26.31 while the strength of the secondary one was only weakly affected by aging: SS=13.74. We demonstrate that the asymmetry between the strength of attractors significantly contributes to SVVE made by seniors at large frame tilts. We hypothesize that a variant VR-RFT may be used in rehabilitation to reduce excessive VFD.

The electric field dependence of single electron emission in the PIXeY two-phase xenon detector

Dual phase xenon detectors are widely used in experimental searches for galactic dark matter particles. The origin of single electron backgrounds following prompt scintillation and proportional scintillation signals in these detectors is not fully understood, although there has been progress in recent years. In this paper, we describe single electron backgrounds in 83mKr calibration events and their correlation with drift and extraction fields, using the Particle Identification in Xenon at Yale (PIXeY) dual-phase xenon time projection chamber. The single electron background induced by the Fowler-Nordheim (FN) effect is measured, and its electric field dependence is quantified. The photoionization of grids and impurities by prompt scintillation and proportional scintillation also contributes to the single electron background.

Laterally Grown ZnO Nanowires for Sensing Applications

<p>Zinc oxide nanowires are a semiconducting material that has many uses in electronic applications. In particular, ZnO nanowires have been used in field effect transistors and applied as sensors for the detection of gases, biomolecules, UV light and as pressure sensors. ZnO nanowires can be fabricated using many different methods, but most require the use of high tempertures and have extensive setup costs. Hydrothermal growth, however, provides a cheap and low temperture method for growing ZnO nanowires. Much work has been done on the synthesis and charcetristaion of ZnO nanowires grown using hydrothermal growth, in partiuclar for photovoltaic applications. Little work has been done on the performace of hydrothermally grown ZnO nanowires in field effect transtors. This thesis looks at applying hydrothermally grown ZnO nanowires as field effect transistors (FET). The FETs are characterised and developed with the intention of using them in senseing applications. The nanowire FET structure is optimised for sensing by developing a method that constrains the nanowires to exclusively lateral growth. A Ti capping layer is fabricated on top of a ZnO seed layer. The ZnO seed layer is then etched with dilute acid so that the Ti layer overhangs the ZnO. This acts as a physical barrier to vertical wire growth from the ZnO seed layer. The maximum deviation of the nanowires from the horizontal can be controlled by etching for different times. Two device types are fabricated using different size nanowires. One uses large nanowires, or nanorods (diameter 400 nm), while the second device type uses a hybrid structure of large nanorods with much thinner nanowires (diameter 20 nm) growing off them. Both device types are characterised as FETs in dry conditions and also when immersed in a number of different liquids. Two different gating setups are also used with the Si/SiO₂ substrate used as a backgate and a Ag/AgCl electrode inserted into liquid as a topgate. The large nanorods only show field dependence when wet due to the large capacitance of the elctric double layer and enhanced band bending. The wet nanorods can achieve on/off ratios of 10³. In contrast, the thinner nanowires show field dependence both when dry and when wet. On/off ratios of more than 10⁴ are achieved. In general the nanowires have superior on off ratios and smaller off current due to their larger surface to volume ratio. Attempts are made to functionalise the nanowires with aptamers so that they can be used as a biosensor. The functionalisation procedure is documented, however the overall procedure proves to be unsuccessful due to the instability and dissolution of the nanowires in tris buffer. The rate of decay in buffer solution is investigated. Both device types are also tested as gas sensors for humidity and ethanol detection. The nanorods show no apparent detection, while the nanowires show some response to ethanol. Further development of the experimental setup is necessary to better characterise the devices. Finally future work on these nanowires is discussed and possible improvements proposed for future development as biosensors and gas sensors.</p>

ZnO nanowires and their application in field-effect transistors

<p>ZnO nanowires have shown great promise as a semiconducting material for a variety of different electronic applications at the nanoscale, and can be easily synthesised at low temperatures using the hydrothermal growth method. However, efforts to reliably produce field-effect transistors (FETs) using ZnO nanowires have been hampered by excessive charge carriers, requiring high temperature annealing (≥400°C) at the expense of the low-temperature synthesis before field dependence is achieved. This thesis presents hydrothermally synthesised ZnO nanowires which can effectively be used as FETs in dry and liquid environments without requiring any annealing or post-growth processing. The role of polyethylenimine (PEI) in the hydrothermal growth of vertical ZnO nanowires is thoroughly investigated. PEI is a polymer used to increase the aspect ratio of ZnO nanowires, but the molecular weight of the polymer and interactions with other growth precursors are often overlooked. Using 4 mM of PEI(MW = 1300 g/mol) results in hierarchical nanowires, consisting of large primary nanowires which abruptly terminate in thinner secondary nanowires. The secondary nanowires, with lengths of up to 10 m and diameters below 50 nm, are synthesised during a PEI-mediated secondary growth phase, where Zn-PEI complexes continue to provide Zn²⁺ ions after the bulk of the precursors have been exhausted. The PEI-mediated synthesis of hierarchical nanowires is used to fabricate FETs by laterally growing intersecting networks of nanowires from spaced pairs of ZnO/Ti films, which have been patterned on SiO₂/Si device substrates. All of these FETs show marked field dependence between VG = -10 V to 10 V, despite being used without annealing. Typical on-off ratios are between 10³ - 10⁵, with threshold voltages between -7.5 V to 5 V. This is a significant result, as the majority of ZnO nanowire FETs reported in the literature require high temperature annealing. Persistent photoconductivity measurements indicate that surface states on the nanowires contribute to the intrinsic field dependence of the devices. Hierarchical nanowires are also synthesised by modular primary and secondary hydrothermal growths. FETs fabricated using these hierarchical nanowires show less field dependence than PEI-mediated hierarchical nanowires, with limited function ality when used in air. The best FET measured in air operates with an on-off ratio of 10⁴ and a threshold voltage of ~ 0 V. Devices which are field-independent in air can be reliably gated by measuring the FETs in a wet environment, using de-ionised water as a dielectric. A back-gated wet FET operates with an on-off ratio of 105 and a threshold voltage of ~ 8 V. Top-gated wet FETs operate with on-off ratios within 103 - 104, and threshold voltages within 0.4 - 0.9 V. These devices also have significantly low subthreshold swings, on the order of 80 mV/decade. FETs are fabricated by contacting individual ZnO nanowires using electron-beam lithography, although only one vertical ZnO nanowire shows field dependence, with an on-off ratio of 10⁴ and a threshold voltage of -7 V. A PEI-mediated hierarchical nanowire is also contacted and shows field dependence, with an on-off ratio of 10² and a threshold voltage of -6 V. The poor on-off ratio is caused by high leakage currents of the device. The contacted nanowires undergo dissolution over time, disappearing from the substrates after 8 months, and also exhibit a conducting-to-insulating transition over 48 hours. This transition can be temporarily reversed by exposure to an electron beam. Neither of these effects are reported in the literature, and their causes are speculated on. Finally, the thesis concludes with proposals for future work to further the advances made here.</p>

Laterally Grown ZnO Nanowires for Sensing Applications

<p>Zinc oxide nanowires are a semiconducting material that has many uses in electronic applications. In particular, ZnO nanowires have been used in field effect transistors and applied as sensors for the detection of gases, biomolecules, UV light and as pressure sensors. ZnO nanowires can be fabricated using many different methods, but most require the use of high tempertures and have extensive setup costs. Hydrothermal growth, however, provides a cheap and low temperture method for growing ZnO nanowires. Much work has been done on the synthesis and charcetristaion of ZnO nanowires grown using hydrothermal growth, in partiuclar for photovoltaic applications. Little work has been done on the performace of hydrothermally grown ZnO nanowires in field effect transtors. This thesis looks at applying hydrothermally grown ZnO nanowires as field effect transistors (FET). The FETs are characterised and developed with the intention of using them in senseing applications. The nanowire FET structure is optimised for sensing by developing a method that constrains the nanowires to exclusively lateral growth. A Ti capping layer is fabricated on top of a ZnO seed layer. The ZnO seed layer is then etched with dilute acid so that the Ti layer overhangs the ZnO. This acts as a physical barrier to vertical wire growth from the ZnO seed layer. The maximum deviation of the nanowires from the horizontal can be controlled by etching for different times. Two device types are fabricated using different size nanowires. One uses large nanowires, or nanorods (diameter 400 nm), while the second device type uses a hybrid structure of large nanorods with much thinner nanowires (diameter 20 nm) growing off them. Both device types are characterised as FETs in dry conditions and also when immersed in a number of different liquids. Two different gating setups are also used with the Si/SiO₂ substrate used as a backgate and a Ag/AgCl electrode inserted into liquid as a topgate. The large nanorods only show field dependence when wet due to the large capacitance of the elctric double layer and enhanced band bending. The wet nanorods can achieve on/off ratios of 10³. In contrast, the thinner nanowires show field dependence both when dry and when wet. On/off ratios of more than 10⁴ are achieved. In general the nanowires have superior on off ratios and smaller off current due to their larger surface to volume ratio. Attempts are made to functionalise the nanowires with aptamers so that they can be used as a biosensor. The functionalisation procedure is documented, however the overall procedure proves to be unsuccessful due to the instability and dissolution of the nanowires in tris buffer. The rate of decay in buffer solution is investigated. Both device types are also tested as gas sensors for humidity and ethanol detection. The nanorods show no apparent detection, while the nanowires show some response to ethanol. Further development of the experimental setup is necessary to better characterise the devices. Finally future work on these nanowires is discussed and possible improvements proposed for future development as biosensors and gas sensors.</p>

ZnO nanowires and their application in field-effect transistors

<p>ZnO nanowires have shown great promise as a semiconducting material for a variety of different electronic applications at the nanoscale, and can be easily synthesised at low temperatures using the hydrothermal growth method. However, efforts to reliably produce field-effect transistors (FETs) using ZnO nanowires have been hampered by excessive charge carriers, requiring high temperature annealing (≥400°C) at the expense of the low-temperature synthesis before field dependence is achieved. This thesis presents hydrothermally synthesised ZnO nanowires which can effectively be used as FETs in dry and liquid environments without requiring any annealing or post-growth processing. The role of polyethylenimine (PEI) in the hydrothermal growth of vertical ZnO nanowires is thoroughly investigated. PEI is a polymer used to increase the aspect ratio of ZnO nanowires, but the molecular weight of the polymer and interactions with other growth precursors are often overlooked. Using 4 mM of PEI(MW = 1300 g/mol) results in hierarchical nanowires, consisting of large primary nanowires which abruptly terminate in thinner secondary nanowires. The secondary nanowires, with lengths of up to 10 m and diameters below 50 nm, are synthesised during a PEI-mediated secondary growth phase, where Zn-PEI complexes continue to provide Zn²⁺ ions after the bulk of the precursors have been exhausted. The PEI-mediated synthesis of hierarchical nanowires is used to fabricate FETs by laterally growing intersecting networks of nanowires from spaced pairs of ZnO/Ti films, which have been patterned on SiO₂/Si device substrates. All of these FETs show marked field dependence between VG = -10 V to 10 V, despite being used without annealing. Typical on-off ratios are between 10³ - 10⁵, with threshold voltages between -7.5 V to 5 V. This is a significant result, as the majority of ZnO nanowire FETs reported in the literature require high temperature annealing. Persistent photoconductivity measurements indicate that surface states on the nanowires contribute to the intrinsic field dependence of the devices. Hierarchical nanowires are also synthesised by modular primary and secondary hydrothermal growths. FETs fabricated using these hierarchical nanowires show less field dependence than PEI-mediated hierarchical nanowires, with limited function ality when used in air. The best FET measured in air operates with an on-off ratio of 10⁴ and a threshold voltage of ~ 0 V. Devices which are field-independent in air can be reliably gated by measuring the FETs in a wet environment, using de-ionised water as a dielectric. A back-gated wet FET operates with an on-off ratio of 105 and a threshold voltage of ~ 8 V. Top-gated wet FETs operate with on-off ratios within 103 - 104, and threshold voltages within 0.4 - 0.9 V. These devices also have significantly low subthreshold swings, on the order of 80 mV/decade. FETs are fabricated by contacting individual ZnO nanowires using electron-beam lithography, although only one vertical ZnO nanowire shows field dependence, with an on-off ratio of 10⁴ and a threshold voltage of -7 V. A PEI-mediated hierarchical nanowire is also contacted and shows field dependence, with an on-off ratio of 10² and a threshold voltage of -6 V. The poor on-off ratio is caused by high leakage currents of the device. The contacted nanowires undergo dissolution over time, disappearing from the substrates after 8 months, and also exhibit a conducting-to-insulating transition over 48 hours. This transition can be temporarily reversed by exposure to an electron beam. Neither of these effects are reported in the literature, and their causes are speculated on. Finally, the thesis concludes with proposals for future work to further the advances made here.</p>