Aperture-Type Near-Field Microscopy Techniques for Probing the Optoelectronic and Thermal Properties of Nanostructured Materials
Abstract
With the development of nanostructured materials for power electronics and renewable energy, there is an increasing demand for probing the photophysical and thermal properties of these systems at the nanoscale, where optical microscopy is normally limited by diffraction. Examples of specific optical and thermal properties from inhomogeneous systems that would be desirable to probe at nanoscale resolution include exciton diffusivity, electroluminescence, and linear heat expansion coefficient. Scanning near-field optical microscopy (SNOM) is a super-resolution imaging technique that uses highly localized evanescent waves to probe light-matter interaction at the nanoscale. In my thesis, I have developed phase-modulated SNOM based methods to probe the exciton diffusion length and electroluminescence of mesoscopically inhomogeneous systems incorporating persistently luminescent materials and photoactive polymers. Thanks to the exceptional nanoscale resolution of SNOM, my work has proven for the first time that long-lasting charges in rare-earth doped strontium aluminate, (arguably the most frequently used persistently luminescent material to date) are diffusive in nature, and not static, as previously assumed. With a similar approach, and using SNOM in both transmission and collection mode, I have also shown that modulated near-field optical techniques are capable of distinguishing electroluminescence from field emission at the surface of thin-film organic light-emitting devices, which will further their understanding. Finally, in the last chapters of my thesis, I introduce near-field thermal expansion imaging, a contactless technique that measures the thermal expansion coefficient on the nanoscale. My method provides the distribution of thermal expansivity over a small region which will be critical for studying the compatibility of interfacing distinct polymeric and/or carbon-based materials in integrated electronic devices, with the expectation of minimizing circuitry failure due to the induced thermal stress created by thermal buildup. Furthermore, I have also shown that our thermal expansion method can be useful for probing the thermal properties of carbon-polymer cyrogels exhibiting negative differential resistance characteristics which have applications in varactors, Gunn diodes and several other electrical devices. Collectively, the work from this thesis has led to a new generation of aperture-type near-field technology that will be beneficial for advancements in studying solid-state thin-films for applications in powered electronics and sustainable and renewable energy applications.