Preparation and Properties of Nano ZnO Dye Sensitized Solar Cells on (Titanium) Substrates

Nowadays, the development of science and technology has been increasing demand for energy. Energy problem has become a bottleneck to restrict the development of international social economy. People pay more and more attention to the development and research of renewable resources. Solar energy is a kind of renewable resource with great potential and no pollution. The commercialized solar cells are mainly silicon solar cells, among which the conversion efficiency of single silicon solar cells is the highest, but the cost of silicon solar cells is high. Therefore, people have been exploring new materials, among which titanium based nano ZnO dye sensitized solar cells have been paid more and more attention by scientists at home and abroad. Based on this, the preparation and performance of nano ZnO dye sensitized solar cells based on titanium are studied. In this paper, the optical anode materials of DSSC are used as the research objects. Three-dimensional ZnO nanoband, one-dimensional graded ZnO nanotube array and one-dimensional sub grade ZnO nanowire array are prepared by anodizing and hydrothermal synthesis. The photovoltaic properties of the three materials are studied. One dimensional graded ZnO, nanotube array films were prepared by two-step hydrothermal synthesis. One dimensional hierarchical ZnO nanowire array is obtained by two-step hydrothermal synthesis. The results show that DSSC is assembled by one-dimensional graded ZnO nanotube array film, and the photoelectric conversion efficiency is 5.1%. Compared with one-dimensional ZnO nanowire array, the efficiency is improved by nearly 90%. The ZnO nanowire of the sub grade is used instead of DSSC The efficiency of photoelectric conversion is only 4% in the photoanode, which is higher than that of the smooth ZnO nanowire photocell.

A Review on the Effects of ZnO Nanowire Morphology on the Performance of Interpenetrating Bulk Heterojunction Quantum Dot Solar Cells

Interpenetrating bulk heterojunction (IBHJ) quantum dot solar cells (QDSCs) offer a direct pathway for electrical contacts to overcome the trade-off between light absorption and carrier extraction. However, their complex three-dimensional structure creates higher requirements for the optimization of their design due to their more difficult interface defect states control, more complex light capture mechanism, and more advanced QD deposition technology. ZnO nanowire (NW) has been widely used as the electron transport layer (ETL) for this structure. Hence, the optimization of the ZnO NW morphology (such as density, length, and surface defects) is the key to improving the photoelectric performance of these SCs. In this study, the morphology control principles of ZnO NW for different synthetic methods are discussed. Furthermore, the effects of the density and length of the NW on the collection of photocarriers and their light capture effects are investigated. It is indicated that the NW spacing determines the transverse collection of electrons, while the length of the NW and the thickness of the SC often affect the longitudinal collection of holes. Finally, the optimization strategies for the geometrical morphology of and defect passivation in ZnO NWs are proposed to improve the efficiency of IBHJ QDSCs.

ZnO nanowire microelectrode arrays for integration with neuronal networks

<p>Microelectrode arrays (MEAs) have been shown as a successful approach for neuroscientists to monitor the signal communication within the neuronal networks for understanding the functionality of the nervous system. However, using conventional planar MEAs is shown to be incapable of precise signal recording from neuronal networks at single-cell resolution due to low signal-to-ratio (SNR). This thesis looks at developing an electronic platform that comprises of zinc oxide nanowires (ZnO-NWs) on MEAs as a future device to record action potential (AP) signals with high SNR from human neuronal networks at single-cell resolution. Specifically, I studied the controlled growth of ZnO nanowires with various morphologies at exact locations across the substrate. I then investigated the biocompatibility of ZnO nanowires with different morphology and geometry for interaction with human NTera2.D1 (hNT) neurons. Finally, I examined the electrical characteristics of MEAs that were integrated with ZnO nanowires and metal encapsulated ZnO nanowires in comparison to the planar MEAs. The hydrothermal growth of ZnO nanowires is thoroughly investigated as a technique to allow synthesis of the nanowires at a low temperature (95°C) with a low cost and high scalability that can also be applied on flexible substrates. The morphology of the ZnO nanowires was varied (diameters of 20–300 nm, lengths of 0.15–6.2 µm, aspect ratios of 6–95 and densities of 10–285 NWs/µm²) by controlling the critical growth parameters such as the precursor concentration (2.5–150 mM), growth time (1–20 h) and additive polyethylenimine (PEI) concentration (0–8 mM). The diameter and length of the ZnO nanowires were increased by increasing the precursor concentration and growth time. Using the standard precursor concentration of 25 mM, growth times of up to 4 h were found effective for the active growth of the nanowires due to the consumption of the precursor ions and precipitation of ZnO. The addition of 6 mM PEI to the growth solution was shown to mediate the growth solution, allowing the extension of the nanowire growth to 20 h or longer. The PEI molecules were also attached to the lateral faces of the nanowires that confined their lateral growth and promoted their axial growth (enhanced aspect ratio from 12 ± 3 to 67 ± 21). Standard photolithography techniques were also introduced to selectively grow ZnO nanowires on exact locations across the substrates. The role of the ZnO seed layer geometry, seed layer area and gap, on the growth of ZnO nanowires was also investigated. Despite using the constant growth parameters (25 mM of precursor concentration with 4 h of growth time) changing the seed line widths (4 µm–1 mm) and the gap between the seed lines (2 µm–800 µm) resulted in the morphology of the nanowires to vary across the same substrate (diameters of 50–240 nm, lengths of 1.2–4.6 µm, aspect ratios of 9–34 and densities of 28–120 NWs/µm²). The seed area ratio of 50% was determined as a threshold to influence the nanowire morphology, where decreasing the seed area ratio below 50% (by increasing the adjacent gap or decreasing the seed layer area) increased the growth rate of the nanowires. The biocompatibility of ZnO nanowires with human hNT neurons was investigated in this work for the first time. The adhesion and growth of hNT neurons on the arrays of ZnO nanowire florets were determined to be influenced by both geometry and morphology of the nanowires. The growth of the hNT neurons was promoted by 30% compared to the control Si/SiO₂ substrate surface when ZnO nanowires with lengths shorter than 500 nm and densities higher than 350 NWs/µm² were grown. The hNT neurons on all nanowires were also demonstrated to be functionally viable as they responded to the glutamate stimulation. ZnO nanowires were shown to improve the electrical properties of the MEAs by reducing the electrochemical impedance due to the increased 3D surface area. The ZnO nanowires that were grown with 50 mM of precursor concentration for 4 h of growth time lowered the impedance from 835 ± 40 kΩ of planar Cr/Au MEAs to 540 ± 20 kΩ at a frequency of 1 kHz. In contrast, the ZnO nanowires that were grown with PEI for 35 h showed that despite the increased surface area by a factor of 45× the impedance was found to be quite high, 2.25 ± 0.2 MΩ at 1 kHz of frequency. The adsorption of PEI molecules to the lateral surfaces of the nanowires was thought to behave as a passivation layer that could have restricted the charge transfer characteristics of the ZnO-NW MEAs. Encapsulation of the pristine ZnO nanowires that were grown with standard precursor concentration of 25 mM for 4 h of growth time with different metallic layers (Cr/Au, Ti and Pt) further improved the electrical characteristics of the MEAs. The ZnO nanowires that were encapsulated with a 10 nm thin layer of Ti and Pt achieved the lowest electrochemical impedance of 400 ± 25 kΩ at 1 kHz in this work. The robustness of the Ti encapsulated ZnO nanowires were also improved in comparison to the PEI ZnO nanowires. The improved electrochemical characteristics and mechanical stability of the MEAs integrated with metal encapsulated ZnO nanowires have shown a great promise for improving the SNR of recording signals from neuronal cells for long term measurements. This work concludes that both pristine ZnO nanowire MEAs and metal encapsulated ZnO nanowire MEAs will be capable of recording AP signals from human neuronal networks at single-cell resolution. However, further optimisation and extensions of the work are required to record AP signals from human neuronal cells.</p>

ZnO nanowire microelectrode arrays for integration with neuronal networks

<p>Microelectrode arrays (MEAs) have been shown as a successful approach for neuroscientists to monitor the signal communication within the neuronal networks for understanding the functionality of the nervous system. However, using conventional planar MEAs is shown to be incapable of precise signal recording from neuronal networks at single-cell resolution due to low signal-to-ratio (SNR). This thesis looks at developing an electronic platform that comprises of zinc oxide nanowires (ZnO-NWs) on MEAs as a future device to record action potential (AP) signals with high SNR from human neuronal networks at single-cell resolution. Specifically, I studied the controlled growth of ZnO nanowires with various morphologies at exact locations across the substrate. I then investigated the biocompatibility of ZnO nanowires with different morphology and geometry for interaction with human NTera2.D1 (hNT) neurons. Finally, I examined the electrical characteristics of MEAs that were integrated with ZnO nanowires and metal encapsulated ZnO nanowires in comparison to the planar MEAs. The hydrothermal growth of ZnO nanowires is thoroughly investigated as a technique to allow synthesis of the nanowires at a low temperature (95°C) with a low cost and high scalability that can also be applied on flexible substrates. The morphology of the ZnO nanowires was varied (diameters of 20–300 nm, lengths of 0.15–6.2 µm, aspect ratios of 6–95 and densities of 10–285 NWs/µm²) by controlling the critical growth parameters such as the precursor concentration (2.5–150 mM), growth time (1–20 h) and additive polyethylenimine (PEI) concentration (0–8 mM). The diameter and length of the ZnO nanowires were increased by increasing the precursor concentration and growth time. Using the standard precursor concentration of 25 mM, growth times of up to 4 h were found effective for the active growth of the nanowires due to the consumption of the precursor ions and precipitation of ZnO. The addition of 6 mM PEI to the growth solution was shown to mediate the growth solution, allowing the extension of the nanowire growth to 20 h or longer. The PEI molecules were also attached to the lateral faces of the nanowires that confined their lateral growth and promoted their axial growth (enhanced aspect ratio from 12 ± 3 to 67 ± 21). Standard photolithography techniques were also introduced to selectively grow ZnO nanowires on exact locations across the substrates. The role of the ZnO seed layer geometry, seed layer area and gap, on the growth of ZnO nanowires was also investigated. Despite using the constant growth parameters (25 mM of precursor concentration with 4 h of growth time) changing the seed line widths (4 µm–1 mm) and the gap between the seed lines (2 µm–800 µm) resulted in the morphology of the nanowires to vary across the same substrate (diameters of 50–240 nm, lengths of 1.2–4.6 µm, aspect ratios of 9–34 and densities of 28–120 NWs/µm²). The seed area ratio of 50% was determined as a threshold to influence the nanowire morphology, where decreasing the seed area ratio below 50% (by increasing the adjacent gap or decreasing the seed layer area) increased the growth rate of the nanowires. The biocompatibility of ZnO nanowires with human hNT neurons was investigated in this work for the first time. The adhesion and growth of hNT neurons on the arrays of ZnO nanowire florets were determined to be influenced by both geometry and morphology of the nanowires. The growth of the hNT neurons was promoted by 30% compared to the control Si/SiO₂ substrate surface when ZnO nanowires with lengths shorter than 500 nm and densities higher than 350 NWs/µm² were grown. The hNT neurons on all nanowires were also demonstrated to be functionally viable as they responded to the glutamate stimulation. ZnO nanowires were shown to improve the electrical properties of the MEAs by reducing the electrochemical impedance due to the increased 3D surface area. The ZnO nanowires that were grown with 50 mM of precursor concentration for 4 h of growth time lowered the impedance from 835 ± 40 kΩ of planar Cr/Au MEAs to 540 ± 20 kΩ at a frequency of 1 kHz. In contrast, the ZnO nanowires that were grown with PEI for 35 h showed that despite the increased surface area by a factor of 45× the impedance was found to be quite high, 2.25 ± 0.2 MΩ at 1 kHz of frequency. The adsorption of PEI molecules to the lateral surfaces of the nanowires was thought to behave as a passivation layer that could have restricted the charge transfer characteristics of the ZnO-NW MEAs. Encapsulation of the pristine ZnO nanowires that were grown with standard precursor concentration of 25 mM for 4 h of growth time with different metallic layers (Cr/Au, Ti and Pt) further improved the electrical characteristics of the MEAs. The ZnO nanowires that were encapsulated with a 10 nm thin layer of Ti and Pt achieved the lowest electrochemical impedance of 400 ± 25 kΩ at 1 kHz in this work. The robustness of the Ti encapsulated ZnO nanowires were also improved in comparison to the PEI ZnO nanowires. The improved electrochemical characteristics and mechanical stability of the MEAs integrated with metal encapsulated ZnO nanowires have shown a great promise for improving the SNR of recording signals from neuronal cells for long term measurements. This work concludes that both pristine ZnO nanowire MEAs and metal encapsulated ZnO nanowire MEAs will be capable of recording AP signals from human neuronal networks at single-cell resolution. However, further optimisation and extensions of the work are required to record AP signals from human neuronal cells.</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>

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>

Fully Vacuum-Sealed Diode-Structure Addressable ZnO Nanowire Cold Cathode Flat-Panel X-ray Source: Fabrication and Imaging Application

A fully vacuum-sealed addressable flat-panel X-ray source based on ZnO nanowire field emitter arrays (FEAs) was fabricated. The device has a diode structure composed of cathode panel and anode panel. ZnO nanowire cold cathodes were prepared on strip electrodes on a cathode panel and Mo thin film strips were prepared on an anode panel acting as the target. Localized X-ray emission was realized by cross-addressing of cathode and anode electrodes. A radiation dose rate of 10.8 μGy/s was recorded at the anode voltage of 32 kV. The X-ray imaging of objects using different addressing scheme was obtained and the imaging results were analyzed. The results demonstrated the feasibility of achieving addressable flat-panel X-ray source using diode-structure for advanced X-ray imaging.