Center for Functional Nanomaterials Seminar

Spatially localized electronic states emerge when disorder reduces electronic scattering length scales close to the Ioffe–Regel limit. Here, I present experimental evidence for Andersonlike localization driven by quantum interference in single-layer graphene with disorder systematically introduced through controlled Ar? ion irradiation. Using inter-defect spacing, LD, obtained from Raman spectroscopy as the ruler of defect distribution, we observed a critical localization threshold near LD∗≈20 nm where multiple independent signatures converge. Timeresolved reflectivity measurements show a nonmonotonic evolution of carrier relaxation times near LD∗, indicating the emergence of spatially localized electronic states. Temperature-dependent transport exhibits exponentially increasing resistivity below this threshold, while the Seebeck coefficient approaches saturation, consistent with a transition toward hopping-dominated transport. Phenomenological analysis of the temperature-dependent transport further captures the evolution of the dominant scattering and conduction mechanisms across the localization crossover. Tight-binding calculations independently identify a crossover from delocalized to exponentially localized states near the Ioffe–Regel condition, kFl≈1. Interestingly, both the thermoelectric power factor and figure of merit exhibit maxima near the localization threshold, demonstrating how disorder-induced energy filtering near mobility edges can enhance thermoelectric response. Building on these results, I will also discuss my ongoing efforts to transition from random distribution of defects to deliberately engineered periodic defect landscapes using focused ion beam patterning and nanoscale materials characterization. Preliminary tight-binding simulations of ordered defect configurations suggest that spatial organization of defects can preserve delocalized electronic states even at relatively high defect densities, motivating our efforts to understand how defect geometry and periodicity can be used to control electronic transport. I will also present the design and development of a cryogenic ultrafast photoconductivity platform integrating femtosecond excitation, microfabricated devices, and high-bandwidth GHz electrical detection to investigate nonequilibrium transport. Together, these studies establish a framework for connecting controlled defects and material structure with quantum transport and relaxation, with potential relevance to understanding how microscopic disorder, interfaces, and fabrication-induced defects influence loss mechanisms in superconducting quantum materials.