New electronic memory device concepts based on metal oxide-polymer nanostructures planer diodes

Nanostructure silver oxide thin films diodes can exhibit resistive switching effects. After an electroforming process the device can be programmed between a low conductance (off-state) and high conductance (on- state) with a voltage pulse and they are already being considered for non-volatile memory applications. However, the origin of programmable resistivity changes in a network of nanostructure silver oxide embedded in polymer is still a matter of debate. This work provides some results on a planer diode which may help to elucidate resistive switching phenomena in nanostructure metal oxide diodes. The XRD pattern after switching appears with different crystalline planes, plus temperature dependent studies reveal that conduction of both on and off states is weak thermal activated. Intriguing the carrier transport is the same for both on and off-states. Difference between states comes from the dramatic changes in the carrier density. The main mechanism of charge transport for on-state is tunneling. The charge transport leads to SCLC in higher voltages pulse for the off state. The mechanism will be explained based on percolation concepts

Electrical characterization of metal-oxide-polymer devices for non-volatile memory applications

The objective of this thesis is to study the properties of resistive switching effect based on bistable resistive memory which is fabricated in the form of Al2O3/polymer diodes and to contribute to the elucidation of resistive switching mechanisms. Resistive memories were characterized using a variety of electrical techniques, including current-voltage measurements, small-signal impedance, and electrical noise based techniques. All the measurements were carried out over a large temperature range. Fast voltage ramps were used to elucidate the dynamic response of the memory to rapid varying electric fields. The temperature dependence of the current provided insight into the role of trapped charges in resistive switching. The analysis of fast current fluctuations using electric noise techniques contributed to the elucidation of the kinetics involved in filament formation/rupture, the filament size and correspondent current capabilities. The results reported in this thesis provide insight into a number of issues namely: (i) The fundamental limitations on the speed of operation of a bi-layer resistive memory are the time and voltage dependences of the switch-on mechanism. (ii) The results explain the wide spread in switching times reported in the literature and the apparently anomalous behaviour of the high conductance state namely the disappearance of the negative differential resistance region at high voltage scan rates which is commonly attributed to a “dead time” phenomenon which had remained elusive since it was first reported in the ‘60s. (iii) Assuming that the current is filamentary, Comsol simulations were performed and used to explain the observed dynamic properties of the current-voltage characteristics. Furthermore, the simulations suggest that filaments can interact with each other. (iv) The current-voltage characteristics have been studied as a function of temperature. The findings indicate that creation and annihilation of filaments is controlled by filling and neutralizing traps localized at the oxide/polymer interface. (v) Resistive switching was also studied in small-molecule OLEDs. It was shown that the degradation that leads to a loss of light output during operation is caused by the presence of a resistive switching layer. A diagnostic tool that predicts premature failure of OLEDs was devised and proposed. Resistive switching is a property of oxides. These layers can grow in a number of devices including, organic light emitting diodes (OLEDs), spin-valve transistors and photovoltaic devices fabricated in different types of material. Under strong electric fields the oxides can undergo dielectric breakdown and become resistive switching layers. Resistive switching strongly modifies the charge injection causing a number of deleterious effects and eventually device failure. In this respect the findings in this thesis are relevant to understand reliability issues in devices across a very broad field.As memórias resistivas baseiam-se na alteração da resistência elétrica de um material ou componente quando submetido a uma tensão elétrica. Este fenómeno deu origem a um novo elemento eletrónico que se passou a designar por “memristor” por sugestão de Leon Chua em 1971. [1] O “memristor” juntou-se assim aos componentes elétricos mais conhecidos, o condensador, a bobine e a resistência. Desde os anos 60 que a comutação resistiva tem sido observada numa variedade de materiais. No contexto desta tese os mais interessantes são por exemplo o SiOx, Al2O3, Ta2O5, ZrO2 e TiO2, onde a comutação resistiva é um processo eletrónico e não envolve uma mudança de fase do material. Os processos físicos envolvidos na comutação de resistência tem permanecido pouco claros. Os vários mecanismos propostos não tem merecido o consenso da comunidade científica. A ausência de um modelo físico tem impedido o desenvolvimento tecnológico destas memórias que têm assim progredido de forma empírica. Apesar da falta de conhecimento sobre os mecanismos físicos, as memórias resistivas oferecem um conjunto de vantagens sobre as tecnologias atuais. Isto despoletou uma intensa atividade de pesquisa quer no meio académico quer pela industria para comercializar este tipo de componente. As memórias resistivas combinam num só componente as vantagens de diversas tecnologias atuais. Podem ter a velocidade de acesso das memórias aleatórias de acesso dinâmico (DRAMs) com um custo muito inferior, com menor consumo de energia e sem necessidade de periodicamente fazer o restauro ou “refeshing”. Oferecem as características não-voláteis de uma memória do tipo “flash”, mas mais robustas, permitindo assim mais ciclos de leitura e escrita. Possibilitam uma elevada densidade e não sofrem dos problemas mecânicos dos discos duros associados com as cabeças de leitura. A comercialização deste tipo de memórias irá revolucionar as tecnologias de informação ao disponibilizar uma elevada capacidade de memória a baixo custo, em dimensões reduzidas e com muito baixo consumo de energia. As memórias resistivas também não precisam de alguma da eletrónica que acompanha os sistemas atuais, nomeadamente os sistemas de “cache”, reduzindo substancialmente os custos e a complexidade dos circuitos. O trabalho desenvolvido nesta tese foi focado nas propriedades elétricas das memórias resistivas com o objetivo de aumentar o nosso conhecimento sobre os mecanismos físicos e elétricos que controlam a comutação resistiva e a velocidade de acesso. As memórias estudadas nesta tese são estruturas do tipo metal-isolador-semicondutor (MIS). Foi usado óxido de alumínio e um polímero conjugado para a camada isolante e semicondutora respetivamente. Estas memórias comutam entre dois estados resistivos diferentes quando submetidas a voltagens definidas durante um certo período de tempo. Paralelamente, foi identificado que o processo físico que conduz a bi-estabilidade elétrica do óxido de alumínio é também responsável pela falha prematura de díodos emissores de luz orgânicos (OLEDs). A presença de óxido de alumínio nativo nos eletrodos dos OLEDs pode dar origem a transições resistivas que alteram o equilíbrio da injeção de portadores de carga e leva a degradação da eletroluminescência. Quer as memórias quer os díodos emissores de luz foram caraterizados usando técnicas elétricas e óticas. Medidas da resposta da corrente a degraus e/ou rampas de tensão permitiram avaliar a velocidade de comutação resistiva. Medidas da impedância no domínio da frequência foram usadas para estudar variações de carga nas interfaces da memória, e por último medidas do ruído elétrico complementadas com medidas óticas permitiram estudar flutuações na corrente causadas pela criação e aniquilação de pequenos caminhos condutores ou filamentos. Todas as medidas foram feitas num grande intervalo de temperatura e frequência. Esta tese contribui para o esclarecimento dos mecanismos físicos que originam comutações entre estados resistivos não-voláteis. As constantes de tempo que controlam o tempo de acesso à memória, isto é, o tempo para ler, escrever ou apagar foram também estudadas. Os resultados obtidos contribuíram para elucidar o mecanismo físico que determina o tempo de acesso. Estratégias para otimizar a rapidez deste tipo de memoria foram propostas. Foi identificado que a condução elétrica é não-homogénea. A corrente é transportada por filamentos. Foi possível quantificar as dimensões físicas e a densidade de corrente transportada por filamentos individuais. O estudo da dinâmica destes filamentos usando técnicas de análise de ruído elétrico permitiu concluir que os filamentos não são criados nem destruídos, mas sim ligados e desligados como interruptores. O mecanismo que liga os filamentos são buracos armadilhados na camada de óxido de alumínio. Quando os buracos são neutralizados por eletrões o filamento é desligado. Este resultado foi um dos contributos mais importantes para a área científica. A condução filamentar dá origem a um conjunto de observações não intuitivas. Concretamente, dá origem a uma dependência anómala da corrente elétrica com a temperatura. A corrente aumenta de forma discreta à medida que a temperatura diminui, isto porque o armazenamento de cargas em armadilhas a baixas temperaturas liga mais filamentos. Adicionalmente, a existência de condução filamentar dá origem a que a corrente elétrica diminua, quando as rampas de tensão rápidas são aplicadas sucessivamente. Os resultados desta tese também sugerem que filamentos de corrente vizinhos podem interatuar e dar origem a fenómenos correlacionados, quer durante o ligar, quer durante o desligar de filamentos. O campo elétrico associado a dois filamentos vizinhos induz um campo elétrico adicional na região intermédia que pode ligar um terceiro filamento. Se um filamento for desligado os filamentos na vizinhança terão mais probabilidade de ser desligados. Simulações usando o “COMSOL Multiphysics” parecem suportar a correlação destes fenómenos. A comutação resistiva é uma propriedade de óxidos binários. Este fenómeno pode ocorrer de forma não intencional, nomeadamente em díodos emissores de luz, células solares, válvulas de spin, transístores de efeito de campo e de uma forma geral, todos os componentes que usam elétrodos que oxidam. O conhecimento adquirido nesta tese é assim relevante para detetar e prevenir problemas de confiabilidade num conjunto vasto de componentes eletrónicos.Dutch Polymer Institute (DPI), Project No. 704, BISTABLE (resistive switching and OLED reliability) and by Fundação para Ciência e Tecnologia (FCT) through the Instituto de Telecomunicacões (IT)

Characterization of Nanomaterials: Selected Papers from 6th Dresden Nanoanalysis Symposiumc

This Special Issue “Characterization of Nanomaterials” collects nine selected papers presented at the 6th Dresden Nanoanalysis Symposium, held at Fraunhofer Institute for Ceramic Technologies and Systems in Dresden, Germany, on 31 August 2018. Following the specific motto of this annual symposium “Materials challenges—Micro- and nanoscale characterization”, it covered various topics of nanoscale materials characterization along the whole value and innovation chain, from fundamental research up to industrial applications. The scope of this Special Issue is to provide an overview of the current status, recent developments and research activities in the field of nanoscale materials characterization, with a particular emphasis on future scenarios. Primarily, analytical techniques for the characterization of thin films and nanostructures are discussed, including modeling and simulation. We anticipate that this Special Issue will be accessible to a wide audience, as it explores not only methodical aspects of nanoscale materials characterization, but also materials synthesis, fabrication of devices and applications

Engineering aperiodic spiral order for photonic-plasmonic device applications

Thesis (Ph.D.)--Boston UniversityDeterministic arrays of metal (i.e., Au) nanoparticles and dielectric nanopillars (i.e., Si and SiN) arranged in aperiodic spiral geometries (Vogel's spirals) are proposed as a novel platform for engineering enhanced photonic-plasmonic coupling and increased light-matter interaction over broad frequency and angular spectra for planar optical devices. Vogel's spirals lack both translational and orientational symmetry in real space, while displaying continuous circular symmetry (i.e., rotational symmetry of infinite order) in reciprocal Fourier space. The novel regime of "circular multiple light scattering" in finite-size deterministic structures will be investigated. The distinctive geometrical structure of Vogel spirals will be studied by a multifractal analysis, Fourier-Bessel decomposition, and Delaunay tessellation methods, leading to spiral structure optimization for novel localized optical states with broadband fluctuations in their photonic mode density. Experimentally, a number of designed passive and active spiral structures will be fabricated and characterized using dark-field optical spectroscopy, ellipsometry, and Fourier space imaging. Polarization-insensitive planar omnidirectional diffraction will be demonstrated and engineered over a large and controllable range of frequencies. Device applications to enhanced LEDs, novel lasers, and thin-film solar cells with enhanced absorption will be specifically targeted. Additionally, using Vogel spirals we investigate the direct (i.e. free space) generation of optical vortices, with well-defined and controllable values of orbital angular momentum, paving the way to the engineering and control of novel types of phase discontinuities (i.e., phase dislocation loops) in compact, chip-scale optical devices. Finally, we report on the design, modeling, and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing, nanoantennas for resonant near-field optical concentration of radiation to individual nanowires, and aperiodic double resonance surface enhanced Raman scattering substrates

Fabrication of a label-free DNA/graphene based electrochemical SNA hybridisation sensor for product authentication and tracing

A thesis submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand. Johannesburg, in fulfilment of the requirements for the Degree of Doctor of Philosophy, August 2017Poor understanding of the interactions at graphene/DNA interfaces has brought tremendous limitations to the development of label-free DNA-graphene based electrochemical/electrical biosensors. The aim of this study was to develop a label-free DNA/graphene-based electrochemical DNA hybridisation sensor, evaluate and benchmark its output electronic signal as a function of the effect of DNA on graphene’s electronic properties. In addition, the study sought to understand the effect of graphene on the nature of DNA. Herein, results of the investigation of the effects of DNA self-immobilisation and subsequent DNA hybridisation on the electronic structure of CVD-grown graphene using a combination of Raman spectroscopy and conductance measurements are presented. A novel UV-Vis spectroscopy dependent measurement technique for the label-free study of the interaction between DNA-graphene interfaces during DNA hybridisation on graphene is reported. Also presented in this work, is a new method of representing electronic events and DNA conformational changes during DNA detection on graphene from current voltage measurements. Non-covalent assembly was used to immobilise single-stranded (ssDNA) probes on CVD-grown graphene. On CVD-grown graphene, Raman peak frequency shifts, intensities and widths of the G and 2D bands after adsorption of the ssDNA probe and its hybridisation with complementary and mismatched ssDNA strands were analysed. The effect of graphene on the structural and conformational changes of DNA upon hybridisation of the ssDNA on the graphene surface both before and after hybridisation with complementary and triple-base mismatch DNA targets were investigated by monitoring UV-Vis absorption peaks at the 200 nm to 300 nm range. The findings were further confirmed through XRD analysis. Using Riemann approximation method, the rate at which the energy is transformed (power) was computed from the area under current-voltage curves.XL201

Properties and Applications of Graphene and Its Derivatives

Graphene is a two-dimensional, one-atom-thick material made entirely of carbon atoms, arranged in a honeycomb lattice. Because of its distinctive mechanical (e.g., high strength and flexibility) and electronic (great electrical and thermal conductivities) properties, graphene is an ideal candidate in myriad applications. Thus, it has just begun to be engineered in electronics, photonics, biomedicine, and polymer-based composites, to name a few. The broad family of graphene nanomaterials (including graphene nanoplatelets, graphene oxide, graphene quantum dots, and many more) go beyond and aim higher than mere single-layer (‘pristine’) graphene, and thus, their potential has sparked the current Special Issue. In it, 18 contributions (comprising 14 research articles and 4 reviews) have portrayed probably the most interesting lines as regards future and tangible uses of graphene derivatives. Ultimately, understanding the properties of the graphene family of nanomaterials is crucial for developing advanced applications to solve important challenges in critical areas such as energy and health

Heterojunction Quantum Dot Solar Cells

The advent of new materials and application of nanotechnology has opened an alternative avenue for fabrication of advanced solar cell devices. Before application of nanotechnology can become a reality in the photovoltaic industry, a number of advances must be accomplished in terms of reducing material and process cost. This thesis explores the development and fabrication of new materials and processes, and employs them in the fabrication of heterojunction quantum dot (QD) solar cells in a cost effective approach. In this research work, an air stable, highly conductive (ρ = 2.94 × 10-4 Ω.cm) and transparent (≥85% in visible range) aluminum doped zinc oxide (AZO) thin film was developed using radio frequency (RF) sputtering technique at low deposition temperature of 250 °C. The developed AZO film possesses one of the lowest reported resistivity AZO films using this technique. The effect of deposition parameters on electrical, optical and structural properties of the film was investigated. Wide band gap semiconductor zinc oxide (ZnO) films were also developed using the same technique to be used as photo electrode in the device structure. Sputtered ZnO thin films with high electron mobility (30 cm2V-1s-1) is achieved at a deposition temperature of 250 °C to eliminate charge collection losses in the solar cell device. The developed AZO and ZnO thin films are a suitable substitute for the expensive indium tin oxide (ITO) and low carrier mobility titanium oxide (TiO2) films respectively. To increase the junction area in the final device, a novel and scalable method was developed for fabrication of upright ordered arrays of ZnO nanowires (NWs) combining top-down etching of ZnO film and silica sphere nanomasking in fluorine-based plasma. The electrical properties of the resulted NWs can be controlled using this approach. The effect of plasma etching parameters and surface treatments on the structural and surface properties of the ZnO NWs were investigated. A high throughput drop-casting process was next developed using colloidal quantum dots (CQDs) for the formation of light absorbing quantum dot film (QD-film) to replace the widely used input-intensive spincoating technique. The QD-film provides controllability over thin film properties via tuning the properties of the CQDs building blocks. Crack-free QD-films were formed with single-step drop casting of CQDs from solution (solution-phase ligand-exchange) and Layer-by-layer (L-B-L) replacement of long ligands with short charge transport-compatible ones (solid-phase ligand-exchange). Charge carrier dynamics were also studied at the CQDs and ZnO interface using photoluminescence decay analyses in order to optimize the selection of CQDs for efficient electron transfer which leads to a better working device. For the first time, the electron transfer in a ZnO/CQD/MoO3 structure was studied to eliminate the effect of other non-radiative pathways and determine the true electron transfer at the CQD/ZnO interface. Effect of CQD size and ligands on the transfer efficiency was discussed. The device concepts and theoretical treatment of QD-film/ZnO were also highlighted to gain insight on the CQD and ZnO optical and electrical parameters affecting the performance of the QD solar cell device. The developed materials and processes were employed for the fabrication of planar and nanowire heterojunction QD solar cell devices. For planar structure power conversion efficiency of η=2.3% at 100 mW/cm2 solar irradiance were achieved

3D mapping of nanoscale physical properties of VCSEL devices

There is clear lack of methods that allows studies of the nanoscale structure of the VCSEL devices1 that are mainly focused on the roughness of the DBR, or using FIB cross-sectioning and TEM analysis of failed devices to observe the mechanism of the degradation. Here we present a recently developed advanced approach that combines Ar-ion nano-cross-sectioning with material sensitive SPM2 to reveal the internal structure of the VCSEL across the whole stack of top and bottom DBR including active area. We report for the first time the direct observation of local mechanical properties, electric potential and conductance through the 3D VCSEL stack. In order to achieve this, we use beam exit cross-section polishing that creates an oblique section with sub-nm surface roughness through the whole VCSEL structure that is fully suitable for the subsequent cross-sectional SPM (xSPM) studies. We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM) 3, surface potential mapping via Kelvin Probe Force Microscopy (KPFM) and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). xSPM allowed to observe the resulting geometry of the whole device, including active cavity multiple quantum wells (MQW), to obtain profiles of differential doping of the DBR stack, profile of electric potential in the active cavity, and spatial variation of current injection in the individual QW in MQW area. Moreover, by applying forward bias to the VCSEL to initiate laser emission, we were able to observe distribution of the potential in the working regime, paving the way to understanding the 3D current flow in the complete device. Finally, we use finite element modelling (FEM) that confirm the experimental results that of the measurements of the local doping profiles and charge distribution in the active area of the VCSEL around the oxide current confinement aperture. While we show that the new xSPM methodology allowed advanced in-situ studies of VCSELs, it establishes a highly efficient characterisation platform for much broader area of compound semiconductor materials and devices. REFERENCES. 1. D. T. Mathes, R. Hull, K. Choquette, K. Geib, A. Allerman, J. Guenter, B. Hawkins and B. Hawthorne, in Vertical-Cavity Surface-Emitting Lasers Vii, edited by C. Lei and S. P. Kilcoyne (2003), Vol. 4994, pp. 67-82. 2. A. J. Robson, I. Grishin, R. J. Young, A. M. Sanchez, O. V. Kolosov and M. Hayne, Acs Applied Materials & Interfaces 5 (8), 3241-3245 (2013). 3. J. L. Bosse, P. D. Tovee, B. D. Huey and O. V. Kolosov, Journal of Applied Physics 115 (14), 144304 (2014)