Mesoscovic magnetic/semiconductor heterostructures

We report the experimental results of Fe and Fe3O4 nanostructures on GaAs(100) surfaces and hybrid Ferromagnetic/Semiconductor/Ferromagnetic (FM/SC/FM) spintronic devices. Element specific x-ray magnetic circular dichroism (XMCD) measurements have shown directly that Fe atoms on the GaAs(100)-4 x 6 surface are ferromagnetic. Within coverages of 2.5 to 4.8 ML superparamagnetic nanoclusters are formed and exhibiting strong uniaxial anisotropy, of the order of 6.0 x 10(5) erg/cm(3). The coercivities of epitaxial Fe dot arrays films grown on GaAs(100) were observed to be dependent on the separation and size of the dots indicating that interdot dipolar coupling affects the magnetization processes in these dots. In addition Fe3O4 films grown on deformed GaAs(100) substrates have been observed to form nanostripes following the topography of the substrate and magneto-optical Kerr effect (MOKE) measurements showed that these nanostripes have uniaxial magnetic anisotropy with easy axis perpendicular to the length of the nanostripes. Meanwhile the FM/SC/FM vertical device has exhibited a biasing current dependent on MR characteristics, with a maximum change of 12% in the MR observed, indicating for the first time a large room temperature spin injection and detection

Nanofabrication

We face many challenges in the 21st century, such as sustainably meeting the world's growing demand for energy and consumer goods. I believe that new developments in science and technology will help solve many of these problems. Nanofabrication is one of the keys to the development of novel materials, devices and systems. Precise control of nanomaterials, nanostructures, nanodevices and their performances is essential for future innovations in technology. The book "Nanofabrication" provides the latest research developments in nanofabrication of organic and inorganic materials, biomaterials and hybrid materials. I hope that "Nanofabrication" will contribute to creating a brighter future for the next generation

Model for porous alumina template formation

Anodized alumina templates have emerged as an important material system for the low cost fabrication of semiconductor and metal nanostructure arrays. This material system utilizes natural self-organization for the creation of periodic arrays of nanoscale structures. The underlying principle is that when aluminum is anodized in a suitable acidic electrolyte under controlled conditions, it oxidizes to form a hydrated aluminum oxide (alumina) containing a two dimensional hexagonal array of cylindrical pores. Due to the excellent periodicity of the pores, and the ability to control the pore diameters, such anodized alumina films can be used as templates for the fabrication of periodic arrays of nanostructures. A process-model based on underlying physics and chemistry of the anodization process is developed. The model developed unravels the interplay of various physical and chemical processes and their dependence on the process parameters such as the electrolyte, temperature, current and voltages and yields, an analytical solution relating the voltage (in constant current anodization) and current (in constant voltage anodization) to time. The predicted time behavior agrees fairly well with experimental observations for sulfuric and oxalic acids. Thus, it is believed that the model is general enough and can be used for any anodization system with changes in appropriate model parameters

Surface studies of organic thin films using scanning probe microscopy and nanofabrication

Porphyrins and metalloporphyrins have unique chemical and electronic properties and thus provide useful model structures for nanoscale studies of the role of chemical structure for electronic properties. Porphyrins have been proposed as viable materials for molecular-based information-storage devices, gas sensors, photovoltaic cells, organic light-emitting diodes and molecular wires. The function and efficiency of porphyrins in devices is largely attributable to molecular architecture and how the molecules are self-organized. Modifications of the porphyrin macrocycle, peripheral groups or bound metal ions can generate a range of electrical, photoelectrical or magnetic properties. The conductive properties are greatly influenced at the molecular level by the organization of porphyrins into supramolecular arrays, aggregates, and nanocrystals on surfaces. Conductive-probe atomic force microscopy (CP-AFM) has been used extensively for studies of alkanes, phenylalkanes and arenethiols; however, the conductive properties of porphyrins have not been studied as rigorously. Characterizations with CP-AFM are becoming prevalent for molecular electronics studies because of the dual capabilities for obtaining physical measurements and structural information with unprecedented sensitivity. For CP-AFM, the tip is placed directly on the sample surface, at a designated force. To acquire current-voltage (I-V) spectra, a conductive tip is grounded, and a bias is applied to the substrate. For this dissertation, cobaltcarborane porphyrins were synthesized using a ring-opening zwitterionic reaction to produce isomers with different numbers of carborane clusters per macrocycle. Particle lithography was used to prepare regular arrangements of well-defined nanopatterns of porphyrin nanocrystals on conductive substrates. Nanopatterned SAMs of alkanethiols and organosilanes were used successfully to direct the nanocrystals of porphyrins on the surface and characterized with contact and tapping mode imaging of AFM. Our goals were to elucidate the role of molecular structure, packing and orientation for the conductive properties of porphyrins. Understanding how the self-organization and surface assembly influence electrical properties and reliable measurements of conductive properties when these molecules are coordinated to different metals and surfaces will provide information for developing predictive models

Scaling and statistics of bottom-up synthesized armchair graphene nanoribbon transistors

Bottom-up assembled nanomaterials and nanostructures allow for the studies of rich and unprecedented quantum-related and mesoscopic transport phenomena. However, it can be difficult to quantify the correlations between the geometrical or structural parameters obtained from advanced microscopy and measured electrical characteristics when they are made into macroscopic devices. Here, we propose a strategy to connect the nanomaterial morphologies and the device performance through a Monte Carlo device model and apply it to understand the scaling trends of bottom-up synthesized armchair graphene nanoribbon (GNR) transistors. A new nanofabrication process is developed for GNR transistors with channel length down to 7 nm. The impacts of the GNR spatial distributions and the device geometries on the device performance are investigated systematically through comparison of experimental data with the model. Through this study, challenges and opportunities of transistor technologies based on bottom-up synthesized GNRs are pinpointed, paving the way to the further improvement of the GNR device performance for future transistor technology nodes

Atomic and close-to-atomic scale manufacturing : status and challenges

Next-generation lithography techniques such as Extreme Ultraviolet Lithography have started to reach their physical limits and will not be able to meet the requirements of future Post-Moore Era Integrated Circuit chips that will be based on quantum, photonic, and DNA computing. These future chips and the next generation of quantum products will require sub-10nm and even atomic-scale functional features. Promising candidates for atomic and close-to-atomic scale manufacturing include well-established tip-based techniques such as Scanning Tunnelling Microscopy (STM) and Atomic Force Microscopy (AFM), however, they suffer from severely low throughput, although parallel tips have been suggested to increase the throughput. The integration of these techniques with others such as AFM in Scanning Electron Microscopy has created new hybrid techniques that have greatly enhanced the capabilities of the standalone process. On the other hand, higher throughput techniques like atomic layer etching (ALE) suffer from poor process control and defects despite being promising candidates due to the self-limiting nature of the processes. Studies into laser processing techniques are being investigated to test the feasibility of laser beam-based atomic scale precision manufacturing. Furthermore, the recent progress in quantum simulations has promoted the development of the optical tweezer towards atomic scale manufacturing

Nanofabrication of metallic and superconducting tungsten-carbon nanostructures using focused ion beams.

La utilización de haces iónicos focalizados (FIBs) para inducir la deposición de distintos materiales es una herramienta altamente versátil para el diseño y fabricación de nnaoestructuras funcionales. En particular, al escanear un haz focalizado de iones de Ga+ o de He+ sobre la superficie de una muestra mientras se inyecta un material precursor en estado gaseoso sobre su superficie, se inducen reacciones de descomposición de éste, que resultan en su deposición parcial en dicha superficie siguiendo el patrón trazado por el FIB. Esta técnica recibe el nombre de deposición inducida por haz iónico focalizado (FIBID).La deposición por Ga+ FIBID del hexacarbonilo de tungsteno da como resultado un material basado en tungsteno y carbono que presenta comportamiento superconductor por debajo de 4.5 K. Junto con la flexiblidad en diseño y crecimiento de estructuras que proporciona el crecimiento por FIBID, este material representa un escenario altamente prometedor para la investigación de distintos fenómenos relacionados con la superconductividad.En esta tesis se muestra de forma experimental que nanohilos de W-C crecidos por Ga+ FIBID pueden albergar transporte no local de vórtices superconductores de largo alcance. Al inyectar una corriente de polarización transversalmente en un extremo del nanohilo cuando éste se encuentra en estado superconductor y bajo la acción de un campo magnético, se ejerce una fuerza de Lorentz sobre los vórtices superconductores allí presentes, que a su vez empujan a los vórtices vecinos y finalmente alcanzan el extremo opuesto del nanohilo. De esta forma, se miden resistencias eléctricas finitas en áreas del nanohilo donde la corriente de polarización no fluye, causadas únicamente por el transporte de vórtices a lo largo del canal superconductor. En relación a este material, se investiga también la obtención de imágenes de la red de vórtices mediante microscopía de efecto túnel, y se investiga la modulación de su corriente crítica mediante la aplicación de un campo eléctrico intenso. Este último fenómeno, similar al efecto de campo basado en la aplicación de un voltaje de puerta, común en el diseño de transistores, se observa experimentalmente en este material por primera vez.La velocidad del proceso de crecimiento por FIBID aumenta drásticamente cuando el proceso se lleva a cabo por debajo de la temperatura de condensación del material precursor. Éste se agrega formando una capa condensada de espesor muy superior a la obtenida al trabajar en fase gaseosa a temperatura ambiente, obteniéndose de esta forma una cantidad de material precursor disponible para su descomposición mucho mayor. Ello permite aumentar el ritmo de crecimiento en un factor de 600. Esta técnica, denominada Cryo-FIBID, da como resultado un material metálico que puede ser uitlizado como constituente de contactos auxiliares para la realización de medidas eléctricas en nanodispositivos.Por último, la resolución lateral en el trazado de nanoestructuras por FIBID puede mejorarse si se utilizan iones de He+ en vez de iones de Ga+. En esta tesis se muestran trabajos relacionados con esta técnica, incluyendo la deposición de estructuras en el plano trazadas con resolución de hastas 10 nm, y la investigación de sus propiedades superconductoras, detectadas por debajo de 2.5-4 K. Se muestra también la capacidad de estos nanohilos para sostener transporte no local de vórtices de largo alcance.<br /