Design, Fabrication and Characterization of Photonic Crystal Light-Emitting Diodes for Solid-State Lighting

Residential, commercial, and industrial lighting applications contribute to ∼19% of total energy consumption worldwide. The application of more efficient sources of lighting, such as solid-state lighting (SSL) sources, could result in potential energy savings of about 65%. Current technologies employ semiconductor-based light-emitting diodes (LEDs) as the core elements of SSL devices to provide general-purpose light in a wide range of color temperatures. However, there still exists several device level issues, such as poor material quality, low quantum efficiencies, large percentage of light being trapped, etc. These non-idealities are barriers for SSL sources replacing incandescent and compact fluorescent sources on an equivalent lumens-per-watt basis.;WVU SSL research interests involve addressing device-level issues associated with III-V nitride materials, as well as optimizing the growth of materials and performance of fabricated devices. One major goal of research efforts is to provide solutions for improvement in light extraction in III-nitride-based devices through the use of integrated, device-level optical elements such as photonic crystals. Photonic Crystals (PhCs) are periodic dielectric structures that possess unique optical properties. PhCs are known for possessing an optical band gap that enables blocking of certain range of wavelengths based on their feature sizes. Additionally, they can also be utilized as diffractive elements when placed in the path of the photons. PhC structures in LEDs are commonly utilized for light extraction improvement and the integration process into the device structure often results in sub-optimal electrical characteristics. The work presented here provides the details of novel processes to add nanophotonic structures to metal and transparent conducting contacts (like indium tin oxide (ITO)) for indium gallium nitride/gallium nitride (InGaN/GaN) based multi-quantum well blue LEDs with emission wavelength in range of lambda=440--470 nm. The developed integration processes will enable improvement in the light extraction of the devices while reducing damage to the active regions of the device and maintaining optimal electrical characteristics. Novel electron beam resist like hydrogen silsesquioxane (HSQ) was utilized to achieve integration of PhCs with minimal degradation. Due to its unique chemical properties, a new classification of PhC structures were realized, that involves cured form of HSQ and named hybrid PhCs. Applying this process, hybrid PhC structures with features of 150 nm in diameter with a pitch of 500 nm in triangular and square lattice configurations fabricated in ITO contacts were integrated into the LEDs. As a result, the devices with hybrid PhC structures showed an improvement of ∼5x in intensity when compared to the unpatterned device.;This work also involved the development of novel bilayer methods using HSQ and sacrificial polymer layers for successful integration of PhCs with holes in transparent conducting layer contacts like ITO. The bilayer process developed will enable in realizing the more traditional PhC structures without the aforementioned process induced sub-optimal electrical characteristics. Additionally, nanosphere lithography (NSL) techniques like spin coating and thermal evaporation were explored as alternative patterning methodologies to enable integration of PhC structures on a large-scale. Utilizing thermal evaporation method, a 98.5% coverage of uniform single layer of polystyrene beads was achieved over a 1.5 x 1.5 cm2 area. This approach to device fabrication will allow PhCs to be integrated into commercial devices inducing less structural damage

Multifunctional vertical interconnections of multilayered flexible substrates for miniaturised POCT devices

Point-of-care testing (POCT) is an emerging technology which can lead to an eruptive change of lifestyle and medication of population against the traditional medical laboratory. Since living organisms are intrinsically flexible and malleable, the flexible substrate is a necessity for successful integration of electronics in biological systems that do not cause discomfort during prolonged use. Isotropic conductive adhesives (ICAs) are attractive to wearable POCT devices because ICAs are environmentally friendly and allow a lower processing temperature than soldering which protects heat-sensitive components. Vertical interconnections and optical interconnections are considered as the technologies to realise the miniaturised high-performance devices for the future applications. This thesis focused on the multifunctional integration to enable both electrical and optical vertical interconnections through one via hole that can be fabricated in flexible substrates. The functional properties of the via and their response to the external loadings which are likely encountered in the POCT devices are the primary concerns of this PhD project. In this thesis, the research of curing effect on via performance was first conducted by studying the relationship between curing conditions and material properties. Based on differential scanning calorimetry (DSC) analysis results, two-parameter autocatalytic model (Sestak-Berggren model) was established as the most suitable curing process description of our typical ICA composed of epoxy-based binders and Ag filler particles. A link between curing conditions and the mechanical properties of ICAs was established based on the DMA experiments. A series of test vehicles containing vias filled with ICAs were cured under varying conditions. The electrical resistance of the ICA filled vias were measured before testing and in real time during thermal cycling tests, damp heat tests and bending tests. A simplified model was derived to represent rivet-shaped vias in the flexible printed circuit boards (FPCBs) based on the assumption of homogenous ICAs. An equation was thus proposed to evaluate the resistance of the model. Vias with different cap sizes were also tested, and the equation was validated. Those samples were divided into three groups for thermal cycling test, damp heat ageing test and bending test. Finite element analysis (FEA) was used to aid better understanding of the electrical conduction mechanisms. Based on theoretical equation and simulation model, the fistula-shape ICA via was fabricated in flexible PCB. Its hollow nature provides the space for integrations of optical or fluidic circuits. Resistance measurements and reliability tests proved that carefully designed and manufactured small bores in vias did not comprise the performance. Test vehicles with optoelectrical vias were made through two different approaches to prove the feasibility of multifunctional vertical interconnections in flexible substrates. A case study was carried out on reflection Photoplethysmography (rPPG) sensors manufacturing, using a specially designed optoelectronic system. ICA-based low-temperature manufacture processes were developed to enable the integration of these flexible but delicate substrates and components. In the manufacturing routes, a modified stencil printing setup, which merges two printing-curing steps (vias forming and components bonding) into one step, was developed to save both time and energy. The assembled probes showed the outstanding performance in functional and physiological tests. The results from this thesis are anticipated to facilitate the understanding of ICA via conduction mechanism and provide an applicable tool to optimise the design and manufacturing of optoelectrical vias

Fabrication and Applications of Printed and Handwriting Electronics

The accelerating arrival of the Internet of Things (IoT) era creates a rapidly growing demand for printed electronic. As a low-cost and green substrate, cellulose paper has become the most attractive choice for the printing of sustainable and disposable electronics. However, manufacture of high quality circuits with high conductivity on cellulose paper remains a challenge due to the substrate’s high porosity and roughness. In this thesis, a method for facile fabrication of hybrid copper-fiber highly conductive features on low-cost cellulose paper with strong adhesion and enhanced bending durability is introduced. With three-dimensional electroless deposition (ELD) of copper, the as-fabricated circuits show ultra-low sheet resistance down to 0.00544 Ω/sq. Taking advantages of the porous structure of paper, together with the precise control of the inkjet droplets, highly conductive vertical interconnected accesses (VIAs) are fabricated for multilayered devices without physically drilling holes or depositing additional dielectric material. To further utilize the unique porous structure of cellulose paper, a scalable fabrication method for flexible, binder-free and all-solid-state supercapacitors is proposed based on the low-cost chemical engraving technique, to construct CuxO nanostructure in-situ on the three-dimensional metallized cellulose fiber structures. Benefitting from both the “2D Materials on 3D Structures” design and the binder-free nature of the fabricated electrodes, substantial improvements to electrical conductivity, aerial capacitance, and electrochemical performance of the resulting supercapacitors (SCs) are achieved, fulfilling the increasing demand of highly customized power systems in the IoT and wearable electronics industries. The above-mentioned work all use inkjet printing for materials deposition. However, as a solvent-based printing technique, inkjet printer has strict requirement of ink properties and suffer from inevitable nozzle clogging. To address these challenges, a fabrication method based on solvent-free laser printing technique is proposed, pushing the manufacture of printed electronics towards an environmentally benign and more cost-efficient manor. Lastly, a one-step react-on-demand (RoD) method for fabricating flexible circuits with ultra-low sheet resistance, enhanced safety and durability is proposed. With the special functionalized substrate, a real-time synthesize of the 3D metal-polymer (3DMP) conductive structure is triggered on demand. The as-fabricated silver traces show an ultralow sheet resistance down to 4 mΩ/sq

Selected Applications of Silicon Nanopillar Arrays.

Interaction of optical waves with nanostructures made of various material systems has been the subject of intensive research for many years. These researches have been mainly driven by the need to make smaller optical devices and exploiting the functionalities offered by light-matter interaction in nanoscale. Majority of the nanostructures are fabricated using electron beam (e-beam) lithography that is slow and expensive. As such alternative methods have been developed to enable nanoscale fabrication faster and less expensive. Among these interferometric lithography (IL) is a relatively simple method for quick fabrication of nanostructures. As IL method generates periodic patterns, exploring the potential applications of the nanostructures that can be fabricated using it, is of primary importance. This dissertation is focused on two applications of silicon nanostructures fabricated by IL method: nanostructured anti-reflection layers (NALs) and plasmonic nanostructures based on arrays of silicon nanopillars (SiNPs) for surface enhanced Raman spectroscopy (SERS). Silicon has been chosen as the structural material due to its extensive usage as the substrate for monolithic electronic circuits and many optical devices. NALs offer several advantages over traditional antireflection coatings made by multilayer deposition. NALs are created by fabricating a nanostructured surface on the substrate material without the need for deposition of different materials and therefore can tolerate large thermal gradients in high power laser applications. We have developed a mathematical model and calculated the optimal profile for the unit cell for a silicon NAL and examined its performance using rigorous coupled-wave analysis (RCWA). The impact of different geometrical parameters on the performance of NALs have been carefully studied. In particular we have evaluated the impact of these geometrical parameters on the transmitted optical power and suppression of higher spatial modes generation. Next using the theoretical outcomes as a guide, we have fabricated several silicon NALs using IL patterning followed by dry etching and measured and computed their performance in mid-IR spectral region. The second category of silicon nanostructures studied here consist of flat top silicon nanopillar (SiNP) arrays with one or a stack of metallic nanodisks on top (with silica nanodisk as spacer) used for Raman enhancement applications. These structures, fabricated using IL, are designed to enhance Raman emission from the adsorbed molecules using surface plasmons. This is achieved by high electric field enhancement, through localization of plasmons at the edges. In order to understand the enhancement mechanism, resonance of these nanostructures along with the E-field enhancements are carefully studied using numerical simulations. Regarding possible role of nanopillars in field enhancement, simulation results have revealed hybridization between SiNP and plasmonic nanodisk stacks. This indicates possibility of transfer of energy of incident laser into plasmonic structure through nanopillar, further amplifying the E-field enhancement. We have also studied the role of geometrical and structural parameters on the field enhancement of these nanostructures. This provides a guide for designing nanostructures with optimal field enhancement for SERS. Next, we have fabricated several samples of SiNPs caped with gold nanodisks and gold-silica-gold nanodisk (stacks) and tested their performance as SERS substrates by measuring the spectrum of the Raman signal (using Thionine and Methylene Blue as target molecules). Our experimental studies have revealed the impact of geometrical parameters of the SiNP and gold nanodisks on the Raman signal. We have also fabricated and tested gold nanodisk performance using silica nanopillar. Finally we have fabricated and tested SiNPs caped with selected non-metallic nanodisks obtained by post processing of nanodisks made of Ge, and TiN. This preliminary study paves the road for a new category of SiNP based SERS substrates that may have advantages over those that use metallic caps in certain applications

Growth and physical-chemical properties of carbon nanotube arrays for energy conversion devices.

Master of Science in Chemistry and Physics. University of KwaZulu-Natal, Durban 2015.Energy demand has been on the increase globally whilst there has been continuous depletion of energy sources. This has prompted investigative research towards sustainable energy through the synthesis of carbon nanotubes (CNTs) for energy storage and conversion devices. Multiwalled carbon nanotubes (MWCNTs) were synthesised using two methods, the thermal chemical vapour deposition (CVD) method and purpose built non-equilibrium plasma-enhanced chemical vapour deposition (PECVD) method. The synthesis temperatures were 850 and 200 °C for CVD and PECVD. In non-equilibrium PECVD the low temperatures used retained the properties of indium tin oxide (ITO) coated glass substrate. MWCNTs synthesis involved the use of either, commercially available ferrocene or synthesised metal nanoparticle catalysts such as iron (Fe), cobalt (Co), nickel (Ni), nickel ferrite (NiFe), nickel cobaltite (NiCo) and cobalt ferrite (CoFe). The metal nanoparticles (MNPs) were synthesised using the co-precipitation method in the presence of hexadecylamine (HDA) as a surfactant. The MNPs and the MWCNTs were characterised using transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and Raman spectroscopy. Growth, physical and chemical properties of the MWCNTs were studied. The synthesised MWCNTs were used as part of the electrode material in organic solar cells (OSC), where poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PDOT: PSS) was used as an electron transporter and poly-3-hexyl thiophene (P3HT) as an electron donor. The OSC performance was tested in a solar simulator. Mono-dispersed MNPs, in the diameter range 3-10 nm, were successfully synthesised. HDA was suitable as both surfactant and reducing agent and it aided the formation of mono-dispersed MNPs. EDX confirmed the presence of typical metals such as Fe, Ni and Co, and the oxygen peak which correlated with the FTIR characteristic metal oxide bonds. The presence of the carbon peak correlated with TGA which showed the decomposition profile of the organic coating. All the CVD methods produced MWCNTs with non-equilibrium PECVD producing vertically aligned MWCNTs directly on the substrate. In non-equilibrium PECVD, liquefied petroleum gas (LPG) and acetylene were successfully used to synthesise MWCNTs at low temperatures. Typical hollow tubular structures of MWCNTs were observed using TEM. These observations correlated morphology from SEM which showed “spaghetti like” structures in the case of thermal CVD and vertical tubular structures in the case of non-equilibrium PECVD. This correlated well with the thermal stability studies of the MWCNTs which showed the characteristic peak for MWCNTs. In addition, Raman spectroscopy showed typical MWCNTs bands, G-band and D-band due to graphitic carbon vibrations and defects respectively and the graphitic nature of the synthesised MWCNTs. The non-equilibrium PECVD, LPG synthesised, MWCNTs were tested in OSC. Positive results that showed dependency on the metal catalyst used during synthesis were observed. The Fe synthesised MWCNTs gave the highest efficiency, 0.116%, among the single metal catalysed MWCNTs followed by Co (0.012%) and Ni with 0.003%. CoFe synthesised MWCNTs also gave the best efficiency (0.009%) among mixed metal catalysed MWCNTs. Therefore, the synthesised MWCNTs gave positive results as part of the electrode material, however, with low efficiencie

Printed and drawn flexible electronics based on cellulose nanocomposites

Sustainability, flexibility, and low-power consumption are key features to meet the growing re- quirements of simplicity and multifunctionality of low-cost, disposable/recyclable smart electronic -of- -based composites hold po- tential to fulfill such demands when explored as substrate and/or electrolyte-gate, or as active channel layer on printed transistors and integrated circuits based on ionic responses (iontronics). In this work, a new generation of reusable, healable and recyclable regenerated cellulose hydro- gels with high ionic conductivity and conformability, capable of being provided in the form of stick- ers, are demonstrated. These hydrogels are obtained from a simple, fast, low-cost, and environ- mental-friendly aqueous alkali salt/urea dissolution method of native cellulose, combined with eration and simultaneous ion incorporation with acetic acid. Their electrochemical properties can be also merged with the mechanical robustness, thermal resistance, transparency, and smooth- - strate. Beyond gate dielectrics, a water-based screen-printable ink, composed of CMC binder and com- mercial zinc oxide (ZnO) semiconducting nanoparticles, was formulated. The ink enables the printing of relatively smooth and densely packed films on office paper with semiconducting func- tionality at room temperature. The rather use of porous ZnO nanoplates is beneficial to form per- colative pathways at lower contents of functional material, at the cost of rougher surfaces. The engineered cellulose composites are successfully integrated into flexible, recyclable, low- voltage (<3.5 V), printed electrolyte-gated office paper or on the ionically modified nanopaper. Ubiquitous calligraphy accessories are used -the- out on the target substrate, where are already printed the devices. Such concept paves the way for a worldwide boom of creativity, where we can freely create personal electronic kits, while having fun at it and without generating waste.Sustentabilidade, flexibilidade e baixo consumo energético são características chave para atender aos crescentes requisitos de simplicidade e multifuncionalidade de sistemas eletrónicos inteligentes de baixo custo, das- Compósitos à base de celulose têm potencial para atender a tais necessidades quando explora- dos como substrato e/ou porta-de-eletrólito ou como camada de canal ativo em transístores impressos e circuitos integrados baseados em respostas iónicas (iontronics). Neste trabalho, é demonstrada uma nova geração de hidrogéis reutilizáveis, reparáveis e recicláveis baseados em celulose regenerada, que apresentam alta condução iónica e conformabilidade, podendo ser fornecidos na forma de adesivos. Estes hidrogéis são obtidos a partir de um método simples, rápido, barato e amigo do ambiente que permite a dissolução de celulose nativa em soluções aquosas com mistura de sal alcalino e ureia, combinado com carboximetil celulose (CMC) para melhorar a sua robustez, seguido da regeneração e simultâneo enriquecimento iónico com ácido acético. As suas propriedades eletroquímicas podem ser combinadas com a inbase de celulose micro/nanofibrilada para obter um substrato eletrolítico semelhante a papel. Para além de portas-dielétricas, foi formulada uma tinta aquosa compatível com serigrafia, composta por CMC como espessante e nanopartículas semicondutoras de ZnO. A tinta permite a impressão de filmes pouco rugosos e densamente percolados sobre papel de escritório, e com funcionalidade semicondutora à temperatura ambiente. O uso alternativo de nanoplacas porosas de ZnO é benéfico para criar caminhos percolativos com menores teores de material funcional, apesar de se obter filmes rugosos. Os compósitos à base celulose foram integrados com sucesso em transístores e portas lógicas porta-eletrolítica, os quais foram impressos em papel de escritório ou no "nanopapel" iconicamente modificado. Acessórios de caligrafia permitem a fácil e rápida padronização de pistas condutoras/resistivas, desenhando-as no substrato alvo, onde estão impressos os dispositivos. Este conceito despoleta um mundo criativo, onde é possível criar livremente kits eletrónicos customizados de forma divertida e sem gerar resíduos

Multi material rectifying device fibers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.Cataloged from PDF version of thesis.Includes bibliographical references.Electronic and optoelectronic device processing is commonly thought to be incompatible with much simpler thermal drawing techniques used in optical fiber production. The incorporation of metals, polymer insulators, and chalcogenide semiconductors into structured fibers has reversed this paradigm and made it possible to realize optoelectronic device functionalities at fiber optic length scales and cost. In spite of the surprising robustness of this processing technique, the electronic performance and complexity of these optoelectronic fiber devices has been constrained by the small set of materials compatible with the fabrication method and the disordered nature of the semiconductor. Specifically, the high density of defects inherent to the amorphous chalcogenide semiconductors precludes the ability to create spatially extended internal electric fields necessary to create more sophisticated devices such as diodes and transistors. In this work, the design, fabrication, and characterization of the first fiber-integrated diode is described. The relevant optical, thermal, and electronic properties of candidate materials compatible with the thermal fiber drawing process are described and measured. Phase changing semiconductors are incorporated into the fiber having both amorphous properties amenable to thermal drawing and crystalline properties ideal for electronic devices. Combinations of metals and semiconductors that form both blocking and non-blocking contacts are identified and combined to form the first diode device that is compatible with the thermal drawing process. Techniques are developed to reduce the dimensions of the resulting devices by an order-of- magnitude compared to all previous multimaterial device fibers.(cont.) A series of measurements of both compositional and potential spatial variation are used to determine that compound formation at specific metal semiconductor interfaces control the rectifying behavior of the fiber integrated rectifying junction. This work demonstrates the ability to synthesize compounds during fiber drawing to create complex electronic structures and combine them to form basic building blocks of circuits into arbitrary long fiber, paving the way to increasingly complex electronic structures and truly intelligent fibers and fabrics.by Nicholas D. Orf.Ph.D