Research and technology highlights of the Lewis Research Center

Highlights of research accomplishments of the Lewis Research Center for fiscal year 1984 are presented. The report is divided into four major sections covering aeronautics, space communications, space technology, and materials and structures. Six articles on energy are included in the space technology section

Self-Healing of Open-Circuit Faults in Organic Thin-Film Transistor-Based Flexible Electronics

Flexible electronics has attracted tremendous attention due to the fast-growing market for portable devices and sensors. However, these devices usually suffer from internal and external forces, which can lead to the failure of interconnects in the circuit. To address this issue, some passive and active strategies have been developed for improving the reliability of interconnects in circuits. Although all these strategies are effective in tackling problems related to the mechanical stress, they have inherent limit in dealing with other failure causes or repairing the open fault. Therefore, in this dissertation, a particle-based self-healing (PBSH) technique has been developed. The detailed study starts with an overall review of the state of the art in techniques developed for improving the reliability of interconnects in flexible circuits. Then the mechanisms of the PBSH technique are analysed and the chemical treatment of metal particles are explored. The physical modelling of the healing is established, and relations between the healing time with the suspension concentration, electric field, length of open gap, and external resistance are verified by experimental results. In addition, to avoid the inherent conductivity of the suspension and the aggregation of micro-particles, they are modified by the oleic acid. This chemical treatment of metal particles improves the uniformity of the suspension and sets a threshold electric field for the occurrence of self-healing. As a key component in flexible circuits, an organic thin-film transistor (OTFT) fabricated by all inkjet-printing process has been developed and characterised in this study. This transistor uses 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPSPentacene) as the semiconductor, poly (vinyl cinnamate) (PVC) as the dielectric, silver as the electrodes, and CYTOP as the encapsulation. Based on this device, two basic circuits: the current mirror and the common-source (CS) amplifier have been successfully built on flexible substrates. The proper functioning of the printed devices and circuits becomes the base for the study of the self-healing in flexible circuits. Subsequently, the PBSH in OTFT device and circuit interconnects is verified. This technique can successfully repair open faults at both source and drain sides in OTFT device interconnects. Simultaneously, it can heal open faults occurring at different locations in flexible current mirror and common-source amplifier circuits biased at the static state. Moreover, the self-healing is also proved to be effective and stable in common-source amplifier when processing alternating current (AC) signals. In order to demonstrate the feasibility of the PBSH technique in real-world applications, more performance characterisations have been conducted. The bending test shows the reliability and stability of the healing in the bent condition. And the dualfaults test verifies the ability of the PBSH technique for healing more than one open fault in interconnects. In addition, methods of packaging the circuit and confining the healing material in the expected region are also developed, which prove the feasibility and manufacturability of the PBSH technique in integrated circuits. This study sets a new benchmark for improving the reliability of inkjet-printed flexible circuits

Micromagnetometry of two-dimensional ferromagnets

The study of atomically thin ferromagnetic crystals has led to the discovery of unusual magnetic behaviour and provided insight into the magnetic properties of bulk materials. However, the experimental techniques that have been used to explore ferromagnetism in such materials cannot probe the magnetic field directly. Here, we show that ballistic Hall micromagnetometry can be used to measure the magnetization of individual two-dimensional ferromagnets. Our devices are made by van der Waals assembly in such a way that the investigated ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from encapsulated graphene. We use the micromagnetometry technique to study atomically thin chromium tribromide (CrBr3). We find that the material remains ferromagnetic down to monolayer thickness and exhibits strong out-of-plane anisotropy. We also find that the magnetic response of CrBr3 varies little with the number of layers and its temperature dependence cannot be described by the simple Ising model of two-dimensional ferromagnetism.Comment: 19 pages, 12 figure