Enhancing Thermoelectric Performance in Single-Crystal-Like Semiconducting Films by Tuning the Carrier Scattering Mechanism

College of Engineering-Interdepartmenta

Enhanced mass activity and stability of bimetallic Pd-Ni nanoparticles on boron-doped diamond for direct ethanol fuel cell applications

In this work, electrochemical deposition of Pd (palladium) and bimetallic Pd-Ni (nickel) nanoparticles on oxygen-terminated boron-doped diamond (BDD) substrate is described for use as electrocatalyst in direct ethanol fuel cell. A potentiostatic two-step electrochemical method involving the electrodeposition of Ni nanoparticles on BDD followed by mono-dispersed Pd nanoparticles was used for the fabrication of Pd-Ni/BDD electrode. The electrocatalytic activity of the bimetallic Pd-Ni nanoparticles was evaluated in an alkaline solution containing ethanol and compared to that of the Pd nanoparticles alone. The bimetallic Pd-Ni nanoparticles showed 2.4 times higher mass activity than the similar systems from literature as well as stability when operated in alkaline media. Higher electrochemical response towards the electrooxidation of ethanol observed for the bimetallic electrocatalysts was due to the synergistic effects of the electron interaction at the interface of the two metals. Chronopotentiometric measurements revealed that Pd is more stable when anchored to the Ni nanoparticles. The optimised loading of mono-dispersed Pd on a foreign Ni metal as nanoparticles plays a crucial role in achieving a high mass (3.63 x 106 mA/g) and specific (10.53 mA/cm2) electrocatalytic activity of Pd towards ethanol electrooxidation in alkaline media

Hut-like pillar array Si Solar Cells

A good texturing pattern for solar cells needs to combine low Reflectance over a large wavelength range and low surface recombination to maximize the output. Nanowire and pyramid patterns offer low Reflectance in the longer and shorter wavelength regions respectively. This paper introduces an exciting new “hut” like texturing pattern with excellent performance, improving that of nanowires and pyramids; over the long and short wavelength ranges respectively. Furthermore, we analyze the effect of different structural parameters such as height, diameter, Surface Coverage (SC) and ϑ, on the solar cell performance

Facile electrochemical synthesis of Pd nanoparticles with enhanced electrocatalytic properties from surfactant-free electrolyte

Synthesis of low-dimensional metallic nanoparticles with a clean surface, high dispersibility, and enhanced atomic surface distribution is extremely important, as these factors strongly influence the electrocatalytic properties of the nanoparticles. In this study, the early stage electrochemical nucleation and growth of palladium nanoparticles (Pd NPs) under potentiostatic control has been investigated on a Au(111) textured substrate. The size distribution and structural characterization of the ex situ as-deposited Pd NPs by means of high-resolution field emission gun-scanning electron microscopy (FEG-SEM) at different stages combined with electrochemical measurements revealed that the cluster of nuclei grew independently through the reduction of metal ions. The electrodeposited Pd NPs were very pure, as confirmed by X-ray photoelectron spectroscopy (XPS), owing to the surfactant-free green electrodeposition process, and they exhibited a highly dispersed average particle size of 2–5 nm. The average nanoparticle size becomes smaller with higher overpotentials for the same deposition time. The synthesized Pd NPs demonstrated the largest specific surface area (four times that of commercial Pd−C) and electrocatalytic activity in ferrocyanide/ferricyanide redox and ethanol electrooxidation processes (35 times that of commercial Pd−C). This work represents an important step in achieving the fundamental understanding of nucleation and growth of nanoparticles correlating the electrocatalytic performances

High speed e-beam writing for large area photonic nanostructures-a choice of parameters

Photonic nanostructures are used for many optical systems and applications. However, some high-end applications require the use of electron-beam lithography (EBL) to generate such nanostructures. An important technological bottleneck is the exposure time of the EBL systems, which can exceed 24 hours per 1 cm2. Here, we have developed a method based on a target function to systematically increase the writing speed of EBL. As an example, we use as the target function the fidelity of the Fourier Transform spectra of nanostructures that are designed for thin film light trapping applications, and optimize the full parameter space of the lithography process. Finally, we are able to reduce the exposure time by a factor of 5.5 without loss of photonic performance. We show that the performances of the fastest written structures are identical to the original ones within experimental error. As the target function can be varied according to different purposes, the method is also applicable to guided mode resonant grating and many other areas. These findings contribute to the advancement of EBL and point towards making the technology more attractive for commercial applications

Осложнения колоректального рака: проблемы и перспективы

КОЛОРЕКТАЛЬНЫЕ НОВООБРАЗОВАНИ

Thermoelectric and structural characterization of individual nanowires and patterned thin films

textThis dissertation presents the development of methods based on microfabricated devices for combined structure and thermoelectric characterizations of individual nanowire and thin film materials. These nanostructured materials are being investigated for improving the thermoelectric figure of merit defined as ZT=S²[sigma]T/K, where S is the Seebeck coefficient, [sigma] is the electrical conductivity, K is the thermal conductivity, and T is the absolute temperature. The objective of the work presented in this dissertation is to address the challenges in the measurements of all the three intrinsic thermoelectric properties on the same individual nanowire sample or along the in plane direction of a thin film, and in correlating the measured properties with the crystal structure of the same nanowire or thin film sample. This objective is accomplished by the development of a four-probe thermoelectric measurement procedure based on a micro-device to measure the intrinsic K, [sigma], and S of the same nanowire or thin film and eliminate the contact thermal and electrical resistances from the measured properties. Additionally the device has an etched through hole that facilitates the structural characterization of the sample using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). This measurement method is employed to characterize individual electrodeposited Bi[subscript 1-x]Te[subscript x] nanowires. A method based on annealing the nanowire sample in a forming gas is demonstrated for making electrical contact between the nanowire and the underlying electrodes. The measurement results show that the thermoelectric propertied of the nanowires are sensitive to the crystal quality and impurity doping concentration. The highest ZT found in three nanowires is about 0.3, which is still lower than that of bulk single crystals at the optimum carrier concentration. The lower ZT found in the nanowires is attributed to the high impurity or carrier concentration and defects in the nanowires. The micro-device is further modified to extend its use to characterization of the in-plane thermoelectric properties of thin films. Existing practice for thermoelectric characterization of thin films is obtaining K in the cross plane direction using techniques such as the 3[omega] method or time domain laser thermal reflectance technique whereas the [sigma] and S are usually obtained in the in-plane direction. However, transport properties of nanostructured thin films can be highly anisotropic, making this combination of measurements along different directions unsuitable for obtaining the actual ZT value. Here, the micro-device is used to measure all three thermoelectric properties in the in-plane direction, thus obtaining the in-plane ZT. A procedure based on a nano-manipulator is developed to assemble etched thin film segments on the micro-device. Measurement results of two different types of thin films are presented in this dissertation. The first type is mis-oriented, layered thin films grown by the Modulated Elemental Reactant Technique (MERT). Three different structures of such thin films are characterized, namely WSe₂, W[subscript x](WSe₂)[subscript y] and (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] superlattice films. All three structures exhibit in-plane K values much higher than their cross-plane K values, with an increased anisotropy compared to bulk single crystals for the case of the WSe₂ film. The increased anisotropy is attributed to the in-plane ordered, cross-plane disordered nature of the mis-oriented, layered structure. While the WSe₂ film is semi-insulating and the W[subscript x](WSe₂)[subscript y] films are metallic, the (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] films are semiconducting with its power factor (S²[sigma]) greatly improved upon annealing in a Se vapor environment. The second type of thin films is semiconducting InGaAlAs films with and without embedded metallic ErAs nanoparticles. These nanoparticles are used to filter out low energy electrons with the introduction of Schottky barriers so as to increase the power factor and scatter long to mid range phonons and thus suppress K. The in-plane measurements show that both the S and [sigma] increase with increasing temperature because of the electron filtering effect. The films with the nanoparticles exhibited an increase in [sigma] by three orders of magnitude and a decrease in S by only fifty percent compared to the films without, suggesting that the nanoparticles act as dopants within the film. On the other hand, the measured in-plane K shows little difference between the films with and without nanoparticles. This finding is different from those based on published cross-plane thermal conductivity results.Mechanical Engineerin

A Study of the Bond and Equity Markets in Greece

EThOS - Electronic Theses Online ServiceGBUnited Kingdo