Microstructure Engineering in Multi-material Fibers

The integration of conducting and semiconducting architectures within thermally drawn thin and flexible fibers is emerging as a versatile platform for smart sensors and imaging systems, medical and biological probes, energy harvesting and advanced textile. Thus far however, fundamental aspects of the microstructure formation and the interplay between microstructure and properties are poorly understood, leading to limited optical, electronic and optoelectronic performances of semiconductor-based fibers. In addition, the integration of metallic glasses, which also entails the control over microstructure and crystallization during the drawing process, has remained unexplored. The first objective of my PhD thesis is hence to shine new light on the microstructure control over fiber-integrated semiconductors, via novel post-drawing schemes and in-depth characterization. A second objective that arises in the course of the project and turns out to progress rapidly with intriguing results, is the integration of metallic glasses within fibers. This time, it is a control of the microstructure during thermal drawing that is highlighted and characterized. Regarding the former, we first compare a regular annealing treatment of the as-drawn fiber with a laser annealing approach to tailor the microstructure of semiconductors in multi-material fibers. By judiciously controlling the laser parameters, we are able to fabricate an electrically addressed polycrystalline semiconductor domain with ultra-large grains, controllable crystallization depth as well as preferentially crystallographic orientations. We then turn to a simple and robust sonochemical approach applied to the amorphous semiconductor at ambient condition without any elevated temperature. The anisotropic surface energy of crystal planes in an organic solvent allows us to control the phase and orientation of monocrystalline nanowires that grow along the desired axis, directly in intimate contact with built-in electrodes. The resulting nanowire-based fiber devices exhibit an unprecedented combination of excellent optical and optoelectronic properties in terms of light absorption, responsivity, sensitivity and response speed that compare favorably with other reported nanoscale planar devices. To highlight the potential of this novel approach, we then demonstrate a fiber-integrated architecture with two nanowire-based devices positioned around a step-index optical fiber, enabling fluorescence imaging using a single multi-functional fiber. Regarding the second objective, we demonstrate, for the first time, the integration of a metallic glass (MG) into multi-material fibers via the thermal drawing approach in the supercooled liquid region. An exquisite control over the capillary-induced instability and crystallization-induced breakup enable us to scale down the thickness of MG to tens of nanometers from the bulk while maintaining their integrity. The crystallization-induced breakup is observed to happen when the thickness of the MG reaches around 40 nm, although this limit could be lowered by proper processing. This simple size-reduction approach indeed provides a unique platform for making nanoscale MG samples with high geometric perfection, enabling the investigation on the nanoscale size effect in crystallization via in-situ heating in a TEM. Moreover, this approach allows for the fabrication of sophisticated fiber architectures, such as MG-rod based fibers, fiber probe with many MG nanowires