Abstract:
Semiconductor nanocrystals (NCs) are nanoscaled crystalline particles with properties determined by their physical dimensions, resulting in unique light-matter interactions. Such quantum-confined crystals can be produced via colloidal synthesis, a method that allows for the preparation of a wide variety of semiconductors with precise control over their composition, size and shape. Two-dimensional (2D) colloidal NCs, in particular, have emerged as an interesting class of these materials and exhibit intriguing optoelectronic properties, which differ from those of their spherical quantum dot (QD) counterparts. Especially 2D nanoplatelets (NPLs), which absorb and emit light in the visible spectrum (e.g. of CdSe or lead halide perovskites), have been intensively studied. Meanwhile, materials that can reach the low-loss optical transmission windows of fiber optics (e.g. lead chalcogenides), located at near-infrared (NIR) wavelengths, remain comparatively less studied despite being sought after as classical or quantum emitters for future glass fiber-based technologies. Early work on colloidal 2D lead chalcogenides has been focused on synthesizing comparatively thick PbS nanosheets, while thin and strongly quantum confined PbS and PbSe NPLs were only recently implemented. This thesis is therefore centered around the target-oriented synthesis and post-synthesis modification of ultrathin 2D PbX (X = S, Se, Te) NCs with tailored optical properties, as well as the investigation and understanding of their photophysics.
Chapters 1 and 2 introduce the topic of colloidal 2D NIR emitters, providing an overview of the basic concepts on which the subsequent chapters are based, and defining the technical terms used throughout this work. In the 3rd chapter, a surface passivation method to increase the photoluminescence quantum yield (PLQY) of PbSe NPLs is established, which is followed by a new synthesis for 2D PbTe NPLs using aminophosphine precursor chemistry and giving access to the “elusive third family member” of 2D PbX (Chapter 4). Chapter 5 reports on a water transfer of 2D PbX NPLs, which preserves their telecom band-friendly NIR PL and opens new possible routes toward functional solid-state nanocomposites. In Chapter 6 and 7, the focus shifts to spectroscopic characterization of 2D PbX by first investigating the low-temperature PL of single 2D PbS NPLs. The cryogenic measurements reveal narrow PL with sub-meV linewidths and a high degree of linear polarization, underscoring the potential of 2D PbS NPLs for quantum optical applications. In Chapter 7 the extreme quantum confinement in PbSe flat QDs (fQDs) is studied via scanning tunneling microscopy and spectroscopy, ensemble cryo-PL, and theoretical tight-binding calculations. Thereby, ultrathin single atomic layer-defined PbSe fQD populations with a thickness down to a monolayer are found, marking the first demonstration of substrate-free PbSe monolayers. These flat quantum dots exhibit extremely strong size quantization in their thickness dimension, while simultaneously being size-quantized by their small lateral extent, thus representing a new class of nanocrystals. Chapter 8 present a method to incorporate colloidal 2D PbSe fQDs into poly(methyl methacrylate) fibers using stable jet electro spinning (SJES). The obtained inorganic-organic hybrid fibers are unidirectionally aligned, have a smooth, well-defined surface, and exhibit the optical properties of the embedded PbSe fQDs. Chapter 7 concludes by summarizing the journey from the initial colloidal synthesis of 2D PbX NCs over their in-depth spectroscopic characterization to the integration of PbSe fQDs into easy-to-handle macroscopic composite materials, as presented in Chapters 3 to 8. Finally, Chapter 10 highlights and discusses possible future research questions.
Nanochemistry