2026 Theses Doctoral

The Optical Properties of Infrared-Active Nanomaterials

Karube, Yuzuka

Near-infrared (NIR) light has broad applications in medicine, bioimaging, and telecommunications; however, the standard detectors and emitters used today are largely inefficient or incompatible with flexible substrates and large-scale solution processable methods. In this work, we explore two solution-processable NIR-active materials: PbS quantum dots (QDs) in a perovskite matrix and erbium-based avalanching upconverting nanoparticles.

Chapters 1 and 2 provide the foundational background for this thesis, introducing fundamental concepts of semiconducting nanocrystals and upconversion in lanthanides, as well as the optical characterization techniques used in subsequent chapters. In addition, Chapter 2 highlights the collaborative projects explored during the course of the PhD, illustrating the broader contributions made beyond the primary projects of the thesis.

Chapter 3 investigates PbS QDs are embedded in an epitaxially grown, lattice matched perovskite at different concentrations and nanocrystal sizes. By controlling QD sizes, the band alignment type is tuned from Type I to Type II. Thin films of these heterostructures are fabricated with a one-step spincoating process using a colloidally stable PbS ink. Absorptance measurements confirm the presence of both materials, while photoluminescence spectra and lifetime measurements indicate charge transfer from the perovskite to the QD for relatively larger nanocrystals. For smaller QDs, the trend reverses, suggesting a switch from Type I to Type II. Ultraviolet photoelectron spectroscopy gives further details of the relative band alignment for the larger sizes. The feasibility of implementing this solution-processable material system is demonstrated through the fabrication of photoconducting devices.

Chapter 4 explores a new class of nonlinear optical materials, erbium-based photon avalanching nanoparticles, through the study of NaErF4 nanoparticles. First, the energy-looping behavior of NaErF₄ is studied by measuring power-dependent luminescence of ensembles and single particles of NaErF₄. Ensemble measurements have much lower thresholds, potentially indicating interparticle interactions, while the single nanoparticle measurements have a higher threshold and a jump-like feature. Time trace measurements reveal photoblinking in single nanoparticles, and temporal correlation analysis shows that fluctuation frequencies have a maximum decay time at the avalanching threshold power. External field-dependence measurements were performed by co-exciting NaErF4₄ with 1550 nm light, which is resonant with the ground state to first excited state transition, showing changes in the power-dependent emission curves in both the film and the single nanoparticle measurements.

Finally, the appendix presents an educational study of the facile synthesis of CsPbBr₃ nanocrystals as a platform for teaching quantum confinement and nanocrystal nucleation and growth kinetics. In collaborations with researchers at ENS Lyon, we explored an ambient condition synthesis procedure of CsPbBr₃ at different concentrations, yielding different sizes of nanocrystals. The slow reaction can be monitored by UV-Vis measurements to investigate the reaction kinetics. In addition, anion exchange is explored in order to demonstrate the dependence of perovskite nanocrystal band gap on halide composition. Through this laboratory module, students are introduced to standard characterization techniques such as UV-Vis absorbance spectroscopy, photoluminescence spectroscopy, and x-ray diffraction.