In this study, we develop self-assembled nano-metal-dots etching mask techniques to fabricate quantum dots (QDs) nano-pillars. We explain the self-assembled nano-metal-dots formation processes by using Dewetting model. Two important experimental factors including (1) interaction force between film and vapor during annealing (£^FV),¡]2¡^interaction force between film and substrate (£^FS) are study to investigate the self-assembled processes. A 200nm thick SiO2 buffer layer is first deposited on the GaAs substrate to congregate thermal energy during the RTA process. In our group, the QDs optimum grown temperature condition is 570¢J, so we develop Au-Ge nano-dots process especially for GaAs based QDs samples. The 8nm thick Au-Ge is annealed at lower 500oC for 60sec under the pressure of 5 E6 Torr to format the nano-dots on QDs samples. The Au-Ge nano-dots have a size and density of 250 ¡Ó 50 nm and 4 E8 cm-2,respectively. We use the Au-Ge nano-dots as mask and dry etching process to fabricate the 9-layer vertical coupled QDs nano-pillars. The diameter and height of the QDs nano-pillar are 250, and 800nm, respectively. According to the QDs density, each nano-pillar contains 1600 QDs in it. The QDs nano-pillar resonance signals are observed by the low temperature cryogenic cathode-luminescence measurement. A strong nano-pillar resonance signal in 1050 nm matched to our simulation results is observed.

abstract: Semiconductor nanostructures are promising building blocks for light management in thin silicon solar cells and silicon-based tandems due their tunable optical properties. The present dissertation is organized along three main research areas: (1) characterization and modeling of III-V nanowires as active elements of solar cell tandems, (2) modeling of silicon nanopillars for reduced optical losses in ultra-thin silicon solar cells, and (3) characterization and modeling of nanoparticle-based optical coatings for light management. First, the recombination mechanisms in polytype GaAs nanowires are studied through photoluminescence measurements coupled with rate equation analysis. When photons are absorbed in polytype nanowires, electrons and holes quickly thermalize to the band-edges of the zinc-blende and wurtzite phases, recombining indirectly in space across the type-II offset. Using a rate equation model, different configurations of polytype defects along the nanowire are investigated, which compare well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects. The presented analysis is a path towards predicting the performance of nanowire-based solar cells. Following this topic, the optical mechanisms in silicon nanopillar arrays are investigated using full-wave optical simulations in comparison to measured reflectance data. The simulated electric field energy density profiles are used to elucidate the mechanisms contributing to the reduced front surface reflectance. Strong forward scattering and resonant absorption are observed for shorter- and longer- aspect ratio nanopillars, respectively, with the sub-wavelength periodicity causing additional diffraction. Their potential for light-trapping is investigated using full-wave optical simulation of an ultra-thin nanostructured substrate, where the conventional light-trapping limit is exceeded for near-bandgap wavelengths. Finally, the correlation between the optical properties of silicon nanoparticle layers to their respective pore size distributions is investigated using optical and structural characterization coupled with full-wave optical simulation. The presence of scattering is experimentally correlated to wider pore size distributions obtained from nitrogen adsorption measurements. The correlation is validated with optical simulation of random and clustered structures, with the latter approximating experimental. Reduced structural inhomogeneity in low-refractive-index nanoparticle inter-layers at the metal/semiconductor interface improves their performance as back reflectors, while reducing parasitic absorption in the metal. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

In this thesis, we use self-assembled nano metal particles as a dry etching mask to from nanopillars. The nanopillars integrated with traditional light-emitting diode (LED) p-type GaN surface is designed to increase the light extraction efficiency. The initial fabrication process adopted in this study is using 100nm SiO2 as thermal aggregation layer. The poor thermal conductivity of SiO2 material will help to accumulate heat on the surface. Then, 10nm Ni thin film is deposited on the SiO2, and rapid annealed at 900oC (working pressure of 1~3¡Ñ10-6 Torr). The Ni nanospheres are prepared to integrate with LED chip processes. We use the etching times (pillar heights) as experimental parameters to study the degree of light extraction efficiency. Traditional right angle branch electrode samples of as grown, 20, 30, 40 sec etching time are analyzed by LI curve measurement. Under 20mA injection current, samples with 20, 30, 40 sec etching times have better light extraction than as grown, an increase of 6.54%, 3.27%, 1.63%, respectively. The experimental results reveal that self-assembled nano metal particles as a dry etching mask on the p-type GaN LED surface can increase the light extraction efficiency.

A better understanding of how the nanoscale environment affects the mechanical properties of materials, in particular metallic nanoparticles and nanocrystalline metals is vital to the development of next generation materials. Of special interest is obtaining a fundamental understanding of the inverse Hall-Petch Effect in nanocrystalline metals, and nanoindentation in individual nanoparticles. Understanding these subjects is critical to understanding how the mechanical properties of materials are fundamentally affected by nanoscale dimensions. These topics have been addressed by a combination of theoretical modeling and in-situ nanoindentation transmission electron microscopy (TEM) analysis. Specifically, the study of the inverse Hall-Petch effect in nanocrystalline metals will be investigated by a thorough review of the literature followed by a proposed novel theoretical model that better explains the experimentally observed behavior of nanocrystalline metals. On the other hand, the nanoindentation of individual nanoparticles is a very new research topic that has yet to aggregate a large body of experimental data. In this context, in-situ TEM nanoindentation experiments on silver nanoparticles will be first performed to determine the mechanisms of deformation in these nanostructures. A theoretical explanation for the observed deformation mechanisms will be then developed and its implications will be discussed. In addition to nanoparticles, this study will also provide unique and valuable insight into the deformation mechanisms of nanopillars, a growing area of research despite much controversy and speculation about their actual mechanisms of deformation. After studying the novel behavior of both nanocrystalline metals and nanoparticles, useful applications of both classes of materials will be explored. The discussion of applications will focus on utilizing the interesting behaviors explored in the dissertation. Of particular interest will be applications of nanoparticles and nanocrystalline materials to coatings, radiation resistance and super-plastic materials. / text

Nanomechanics emerged as chemists and physicists began fabricating nanoscale objects. However, there are some materials that have neither been fabricated nor mechanical investigated at the nanoscale, such as rhodium. Rhodium is used in many applications, especially in coatings and catalysis. To contribute to the understanding the nano-properties of this important material, rhodium was fabricated and mechanically investigated at the nanoscale. The nanopillars approach was employed to study size effects on mechanical properties. Nanopillars with different diameters were fabricated using electroplating followed by uniaxial compression tests. SEM was used as a quality control technique by imaging the pillars before and after compression to assure the absence of buckling, barrelling, or any other problems. Transmission electron microscopy (TEM) and SEM were used as microstructural characterization techniques, and the energy-dispersive X-ray spectroscopy (EDX) was used as the chemical characterization technique. Due to substrate induced effects, only the plastic region of the stress-strain curves were investigated, and it was revealed that rhodium softens with decreased nanopillar diameter. This softening/weakening effect was due to the nanocrystallinity of the fabricated pillars. This effect is consistent with the literature that demonstrates the reversed size effect of nanocrystalline metals, i.e., smaller is weaker. Further studies should focus on eliminating the substrate effect that was due to the adhesion layers between Rh and the silicon substrate being softer than Rh, consequently, causing Rh to sink into the adhesion layer when compressed and thus perturbing the stress-strain curve. Moreover, further investigation of other properties of Rh is required to achieve a comprehensive understanding of Rh at the nanoscale, and to render it suitable for specific, multivariable applications.

The electro-optical properties of the ZnO/Si heterojunction embedded with self-assembled gold nanoparticles on patterned silicon substrate are investigated in this master thesis. High quality n-type ZnO film is deposited on patterned p-type silicon substrate by radio-frequency sputtering to form a ZnO/Si pn junction. The patterned silicon substrates are prepared by ICP-RIE using self-assembled nickel metal dot and silicon dioxide as etching mask. The optimum ICP process conditions of silicon nanopillars are CF4/Ar ~ 40/40 sccm and bias/RF power 400/400 W. Silicon nanopillars of diameter ~ 50 nm and height 100~400 nm are formed on the substrate surface. ZnO film is then deposited of a growth rate ~ 12 nm/min at the substrate temperature = 200oC. The plasmonic effects on the electro-optical properties, including photoluminescence (PL), reflection, and electrical characteristics, are studied by adding self-assembled gold nanoparticles within the ZnO film. The self-assembled gold nanoparticles are formed by thermal deposition and rapid thermal annealing at 700oC. The gold nanoparticles are observed by scanning electron microscopy (SEM) and particles of diameter about 100 nm. The PL intensity of ZnO is enhanced more than ten times at the peak wavelength = 380 nm by adding the gold nanoparticles and silicon nanopillars. Strong blue emission light could be saw with the naked eyes. For the electric characteristics, self-assembled gold nanoparticles in patterned ZnO/Si heterojunction show photoelectric conversion phenomenon because of high electromagnetic absorption and plasmonic effects.

Nanomechanics emerged as chemists and physicists began fabricating nanoscale objects. However, there are some materials that have neither been fabricated nor mechanical investigated at the nanoscale, such as rhodium. Rhodium is used in many applications, especially in coatings and catalysis. To contribute to the understanding the nano-properties of this important material, rhodium was fabricated and mechanically investigated at the nanoscale. The nanopillars approach was employed to study size effects on mechanical properties. Nanopillars with different diameters were fabricated using electroplating followed by uniaxial compression tests. SEM was used as a quality control technique by imaging the pillars before and after compression to assure the absence of buckling, barrelling, or any other problems. Transmission electron microscopy (TEM) and SEM were used as microstructural characterization techniques, and the energy-dispersive X-ray spectroscopy (EDX) was used as the chemical characterization technique. Due to substrate induced effects, only the plastic region of the stress-strain curves were investigated, and it was revealed that rhodium softens with decreased nanopillar diameter. This softening/weakening effect was due to the nanocrystallinity of the fabricated pillars. This effect is consistent with the literature that demonstrates the reversed size effect of nanocrystalline metals, i.e., smaller is weaker. Further studies should focus on eliminating the substrate effect that was due to the adhesion layers between Rh and the silicon substrate being softer than Rh, consequently, causing Rh to sink into the adhesion layer when compressed and thus perturbing the stress-strain curve. Moreover, further investigation of other properties of Rh is required to achieve a comprehensive understanding of Rh at the nanoscale, and to render it suitable for specific, multivariable applications.

This document presents the modelling and characterization of novel photonic devices based on periodic arrays of multiwalled carbon nanotubes. Multiwalled carbon nanotubes are mostly metallic in nature and interesting plasmonic effects are observed when nanotubes are grown close together, with spacing of about 400 nm. The effective electronic mass on the nanotubes changes, due to mutual coupling between them and they start displaying dielectric properties which are inherently different from the their own, forming metamaterials. We present a plasmonic high pass filtering application of carbon nanotube based metamaterials. Some promising modelling and experimental results are demonstrated showing a strong cut-off filtering effect at the plasma frequency displayed by the periodic arrays of multiwalled carbon nanotubes. The artificial negative dielectric constant displayed by the nanotube arrays was also successfully utilised for producing micron-scaled applications like optical waveguides and negative lenses for overcoming the diffraction limit. The fabrication of these optical devices using the arrays of silicon nanopillars was also considered. These arrays when fabricated at nano-scaled dimensions (of about 400 nm) present a greater degree of periodicity and require a simpler fabrication process compared to carbon nanotubes. We report the detailed computational analysis on silicon nanopillars based photonic crystals, waveguides and metamaterials which operate well within in the optical regime. However, due to the fabrication limitations, the fabricated Si nanopillars presented an inverted cone shape profile along their lengths. These inverted nanocone structures were successfully utilised for enhancing reflection from Si surfaces for applications in photovoltaic devices. Lastly we present a novel application of carbon nanotube arrays for producing micron-scale Fresnel lens arrays. Forests of carbon nanotubes were utilised as absorbing media on top of a bare silicon substrate. Optical diffraction of light across the nanotube forests produced strong focusing of light, at focal lengths of order 125 microns. Numerical simulations were in excellent agreement with the measured results.

Semiconductor nanotechnology is today a very well studied subject, and demonstrations of possible applications and concepts are abundant. However, well-controlled mass-fabrication on the nanoscale is still a great challenge, and the lack of nanofabrication methods that provide the combination of required fabrication precision and high throughput, limits the large-scale use of nanodevices. This work aims in resolving some of the issues related to nanostructure fabrication, and deals with development of nanofabrication processes, the use of size-reduction for reaching true nanoscale dimensions (20 nm or below), and finally the optical and electrical characterization to understand the physics of the more successful structures and devices in this work. Due to its widespread use in microelectronics, silicon was the material of choice throughout this work. Initially, a fabrication process based on electron beam lithography (EBL) was designed, allowing controlled fabrication of devices of dimensions down to 30 nm, although, generally, initial device dimensions were above 70 nm, allowing the flexible but low-throughput EBL, to be replaced by state-of-the-art optical lithography in the case of industrialization of the process. A few main processes were developed throughout the course of this work, which were capable of defining silicon nanopillar and nano-wall arrays from bulk silicon, and silicon nanowire devices from silicon-on-insulator (SOI) material. Secondly, size-reduction, as a means of providing access to few-nanometer dimensions not available by current lithography techniques was investigated. An additional goal of the size-reduction studies was to find self-limiting mechanisms in the process, that would limit the impact of variations in the size and other imperfections of the initial structures. Thermal oxidation was investigated mainly for self-limited size-reduction of silicon nanopillars, resulting in well-defined quantum dot arrays of few-nm dimensions. Electrochemical etching was employed to size-reduce both silicon nanopillars and silicon nanowires down into the 10-nm regime. This being a novel application, a more thorough study of electrochemical etching of low-dimensional and thin-layer structures was performed as well as development of a micro-electrochemical cell, enabling electrochemical etching of fabricated nanowire devices with improved control. Finally, the combination of nanofabrication and size-reduction resulted in two successful device structures: Sparse and spatially well-controlled single silicon quantum dot arrays, and electrically connected size-reduced silicon nanowires. The quantum dot arrays were investigated through photoluminescence spectroscopy demonstrating for the first time atomic-like photoemission from single silicon quantum dots. The silicon nanowire devices were electrically characterized. The current transport through the device was determined to be through inversion layer electrons with surface states of the nanowire surfaces greatly affecting the conductance of the nanowire. A model was also proposed, capable of relating physical and electrical properties of the nanowires, as well as demonstrating the considerable influence of charged surface states on the nanowire conductance. / QC 20101101

This study focuses on fabrication of dry adhesives mimicking gecko adhesion. We also look into the origin of the supreme adhesion of geckos, which have inspired the fabrication of fibrillar dry adhesives during the last decade or so. In principle, the superior material properties of ??-keratin (the main material comprising the fibrillar feature on gecko toe pads) along with the hierarchical high aspect-ratio fibrillar structure of geckos??? foot pad have enabled geckos to stick readily and rapidly to almost any surface in both dry and wet conditions. In this research, non-sticky fluoropolymer (Teflon AF) resembling ??-keratin rigidity and having an extremely low surface energy and dielectric constant was applied to fabricate a novel dry adhesive consisting of extremely high aspect-ratio nanopillars (200 nm in diameter) terminated with a fluffy top nanolayer. Both the nanopillars and the terminating layer were fabricated concurrently by replica-molding using a nanoporous anodic aluminum oxide membrane as the mold. In particular, upon infiltration of Teflon AF melt into the anodic aluminum oxide nanopores, the polymer melt fingered over the pore walls. The fingerlike structure formed during infiltration, subsequently collapsed after removal of the mold, developing a unique sheet-like nanostructure on top of the base nanopillars. Concurrent fabrication of the terminating nanostructure helps the fabrication of extremely high aspect-ratio (27.5???225) nanopillars which, up to an aspect-ratio of 185, neither collapse at the tip nor bundle. In order to fabricate nanopillars of different topographical properties, in our first approach, the height of the nanopillars as well as the size and density of the terminating nanostructure are carefully controlled by adjusting the processing temperature. Following that, a novel replica-molding technique for fabrication of bi-level Teflon AF nanopillars is reported. The developed technique relies on the concurrent heating and cooling of the Teflon AF melt which filled vertically-aligned alumina nanochannels. Unlike conventional polymer infiltration methods which consist of filling the mold by only heating the polymer above its glass transition temperature, in our novel method, the polymer melt is also simultaneously cooled down during the infiltration process. Concurrent cooling of the Teflon AF melt allows control over the interfacial instabilities of the polymer thin film, which forms ahead of the polymer melt upon its infiltration into the alumina nanochannels. Doing so, the geometrical properties of the subsequently developed peculiar fluffy nanostructure ??? after removal of the mold ??? on top of the extremely high aspect-ratio Teflon AF nanopillars (~25 ??m tall) are modified. In this project, we have also shown that the adhesion of the fabricated dry adhesives for the most part arises from electrostatic interactions of the applied polymer. In other words, Teflon AF, having an exceptional potential for developing electric charges at its surface upon contact with other materials via the so-called contact electrification phenomenon, can develop significant electrostatic interactions at its surface upon contact. In the current thesis, tribological results were discussed in detail to clarify the contribution of the structural properties of the fabricated dry adhesives toward their remarkable adhesion and friction forces generated via contact electrification. Nanopillars of specific geometrical properties have achieved remarkable adhesion and friction strengths, up to ~2.1 N/cm2 and 17 N/cm2, respectively (up to ~2.1 and 1.7 times larger than those of a gecko toe pad). It is commonly accepted that the adhesive performance of other synthetic bio-inspired dry adhesives is due to the formation of van der Waals interactions at the tip or side of the dry adhesives fibrils with the substrate they are brought into contact with. However, what has been usually neglected in this connection is that electrostatic interactions may also be developed at the contact between any two materials via the familiar contact electrification phenomenon. Although contact electrification is common and can have a large influence on interfacial interaction forces, its impact on adhesive properties of synthetic dry adhesives has been overlooked. Our results on adhesion of bi-level Teflon AF nanopillars, which can generate strong adhesion forces relying on electrostatic interactions arising from contact electrification, have brought to light again the idea that charging the surface of dry adhesives, specifically polymeric ones, can play a very crucial role in their adhesive behavior. From this perspective, the main reasons that have caused this lack of attention to this concept and the possible contributions of contact electrification to interfacial interactions of polymeric dry adhesives, other than bi-level Teflon AF nanopillars, are also thoroughly discussed in this thesis. Besides synthetic fibrillar dry adhesives, the possibility of the occurrence of contact electrification and its contribution to the supreme dry adhesion of geckos have also been overlooked for several decades. In this research, by the simultaneous measurement of electric charges and adhesion forces that gecko toe pads develop on two distinct substrates (a sticky and a non-sticky one), we have shown that the toe pads generate significantly large amounts of electric charge on both substrates. More importantly, we have found that there is a direct correlation between the contact electrification-driven electrostatic forces and the measured adhesion forces. Otherwise stated, we have shown that what makes the difference that geckos stick strongly to one surface and not to the other are the electrostatic interactions arising from contact electrification, and not van der Waals interactions, which have been considered as the prime source of adhesion of geckos for many years.