The behavior of hydrothermally synthesized hematite nanorods prepared on spin coated seed layers

Herein we report on the effect of varied spin-coated seed layer concentrations of Iron (III) chloride hexahydrate (FeCl3.6H2O) on the photoelectrochemical performance of hydrothermally synthesized hematite nanorods. The seed layers were prepared from 0.05, 0.07, 0.09, 0.11, and 0.13 M concentrations of FeCl3.6H2O. The nanorods were vertically aligned with slight inclinations over the seed layers with the two lowest molar concentrations (0.05 and 0.07 M) of FeCl3.6H2O. A further increase in seed layer concentrations transformed the nanorods as they grew over others and agglomerated into clusters. Structural analysis using X-ray diffraction (XRD) and Raman spectroscopy demonstrated uniform hematite crystalline peaks for all the samples. All samples absorbed highly in the visible region within an onset absorption edge wavelength ranging from 624 to 675 nm. Overall, the nanorods synthesized over the lowest seed layer concentration of 0.05 M of FeCl3.6H2O exhibited the highest photocurrent density of 0.077 mA/cm2 at 1.5 V vs. reversible hydrogen electrode. The results obtained provide important information about the structural, optical, and photoelectrochemical properties of hematite nanorods synthesized over varied seed layer concentrations. This is a key contribution in understanding and enhancing the hematite nanorods performance for photocatalytic applications.

Ultrasmooth thin film and nano-scale structure fabrication to extend the bounds of optical microscopy

<p>Plasmonic devices including superlenses, hyperlenses, and far-field superlenses are specially fabricated elements which can improve the resolution of optical microscopy past inherent theoretical limits. However, their fabrication is extremely difficult as they often require ultrasmooth thin films and finely structured silver (Ag). While Ag has an ideal response for these types of lenses, its fabrication in such devices is challenging. Hence, this thesis investigates viable methods of producing ultrasmooth Ag thin films and nano-scale Ag features in order to advance research in plasmonic devices. Optimum plasmon response requires the fabrication of ultrasmooth thin silver films, which presents several challenges, such as high surface roughness and high optical loss. Thin 1 nm seed layers were fabricated in advance of Ag layers in order to improve the surface properties of Ag. We found that a 1 nm germanium (Ge) seed layer results in a 400% reduction in surface roughness down to 0.64 nm(RMS), but offers increased optical loss by about 3% over Ag alone. However, an inert atmosphere high temperature anneal of a Ge/Ag stack results in preferential grain growth, further reducing surface roughness to 0.61 nm(RMS), while also improving transmission by up to 14% over Ag alone. Similar procedures were conducted on copper (Cu) and silver oxide (AgOx) seed layers. While Cu results in very smooth Ag films of 0.61 nm(RMS) for films < 10 nm thick, performance deteriorates at Ag thicknesses above 10 nm, which are preferred for the plasmonic applications identified above. Furthermore, AgOx produces very rough surfaces on substrates which are amorphous–a property which is essential for our use. However, AgOx on crystalline substrates produced smooth surfaces of 0.3 nm(RMS) and may be useful for other plasmonic applications. Interference lithography (IL) was selected as the method to create the periodic nano-scale structures. The IL equipment was modified with the addition of bandpass light filters, 5 μm pinhole Fourier filters, and air vortex shields. Also, elimination of both external vibration and time-dependant vacuum lines are included. With this IL environment, we were able to produce periodic gratings anywhere from 1 μm-300 nm pitch through rigorous optimisation of photoresist, exposure, and development processes. Ultimately, this lead to the fabrication of high contrast, 200 nm period gratings for use in a far-field superlens. The designs and procedures outline within will result in increased performance and production of far-field superlenses with limited equipment, therefore facilitating increased performance of optical microscopes.</p>

Ultrasmooth thin film and nano-scale structure fabrication to extend the bounds of optical microscopy

<p>Plasmonic devices including superlenses, hyperlenses, and far-field superlenses are specially fabricated elements which can improve the resolution of optical microscopy past inherent theoretical limits. However, their fabrication is extremely difficult as they often require ultrasmooth thin films and finely structured silver (Ag). While Ag has an ideal response for these types of lenses, its fabrication in such devices is challenging. Hence, this thesis investigates viable methods of producing ultrasmooth Ag thin films and nano-scale Ag features in order to advance research in plasmonic devices. Optimum plasmon response requires the fabrication of ultrasmooth thin silver films, which presents several challenges, such as high surface roughness and high optical loss. Thin 1 nm seed layers were fabricated in advance of Ag layers in order to improve the surface properties of Ag. We found that a 1 nm germanium (Ge) seed layer results in a 400% reduction in surface roughness down to 0.64 nm(RMS), but offers increased optical loss by about 3% over Ag alone. However, an inert atmosphere high temperature anneal of a Ge/Ag stack results in preferential grain growth, further reducing surface roughness to 0.61 nm(RMS), while also improving transmission by up to 14% over Ag alone. Similar procedures were conducted on copper (Cu) and silver oxide (AgOx) seed layers. While Cu results in very smooth Ag films of 0.61 nm(RMS) for films < 10 nm thick, performance deteriorates at Ag thicknesses above 10 nm, which are preferred for the plasmonic applications identified above. Furthermore, AgOx produces very rough surfaces on substrates which are amorphous–a property which is essential for our use. However, AgOx on crystalline substrates produced smooth surfaces of 0.3 nm(RMS) and may be useful for other plasmonic applications. Interference lithography (IL) was selected as the method to create the periodic nano-scale structures. The IL equipment was modified with the addition of bandpass light filters, 5 μm pinhole Fourier filters, and air vortex shields. Also, elimination of both external vibration and time-dependant vacuum lines are included. With this IL environment, we were able to produce periodic gratings anywhere from 1 μm-300 nm pitch through rigorous optimisation of photoresist, exposure, and development processes. Ultimately, this lead to the fabrication of high contrast, 200 nm period gratings for use in a far-field superlens. The designs and procedures outline within will result in increased performance and production of far-field superlenses with limited equipment, therefore facilitating increased performance of optical microscopes.</p>

Low-Temperature Growth of ZnO Nanowires from Gravure-Printed ZnO Nanoparticle Seed Layers for Flexible Piezoelectric Devices

Zinc oxide (ZnO) nanowires (NWs) are excellent candidates for the fabrication of energy harvesters, mechanical sensors, and piezotronic and piezophototronic devices. In order to integrate ZnO NWs into flexible devices, low-temperature fabrication methods are required that do not damage the plastic substrate. To date, the deposition of patterned ceramic thin films on flexible substrates is a difficult task to perform under vacuum-free conditions. Printing methods to deposit functional thin films offer many advantages, such as a low cost, low temperature, high throughput, and patterning at the same stage of deposition. Among printing techniques, gravure-based techniques are among the most attractive due to their ability to produce high quality results at high speeds and perform deposition over a large area. In this paper, we explore gravure printing as a cost-effective high-quality method to deposit thin ZnO seed layers on flexible polymer substrates. For the first time, we show that by following a chemical bath deposition (CBD) process, ZnO nanowires may be grown over gravure-printed ZnO nanoparticle seed layers. Piezo-response force microscopy (PFM) reveals the presence of a homogeneous distribution of Zn-polar domains in the NWs, and, by use of the data, the piezoelectric coefficient is estimated to be close to 4 pm/V. The overall results demonstrate that gravure printing is an appropriate method to deposit seed layers at a low temperature and to undertake the direct fabrication of flexible piezoelectric transducers that are based on ZnO nanowires. This work opens the possibility of manufacturing completely vacuum-free solution-based flexible piezoelectric devices.