Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Abstract

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including “homogeneous” broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II–VI quantum dots (QDs) and nanoplatelets, III–V QDs, alloyed QDs, metal–halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Understanding Hot Injection Quantum Dot Synthesis Outcomes Using Automated High-Throughput Experiment Platforms and Machine Learning

“…The cascade model also demonstrates a slight but noticeable reduction in prediction errors compared to the direct model due to the correlation between fwhm and peak energy (Figure c): particles display lowered homogeneous and inhomogeneous line width as they increase in size. Homogeneous peak broadening stems from the intrinsic properties of individual semiconductor nanoparticles while inhomogeneous peak broadening stems from the heterogeneity of particle properties in an ensemble (e.g., size dispersion) . Generally, the homogeneous line width of smaller particles is higher due to the increased coupling between the exciton and surface phonon modes .…”

“…Semiconductor quantum dots (QDs) offer the potential to design nanomaterials with widely tunable optoelectronic properties by control over size, shape, phase, and composition. , They demonstrate promise in a wide range of applications such as catalysis, coatings, imaging, sensing, displays, photovoltaics, and more. ,− In many of these applications, tight control over QD properties (shape, size distribution, composition, and heterostructures) is critical as these parameters determine key optical and electronic properties including absorption and emission. Several studies have sought to understand QD synthesis by uncovering mechanisms of their formation (reaction, nucleation, and growth steps) and by demonstrating methods to tune synthesis parameters to achieve specific QD properties. − …”

Ligand Desorption and Fragmentation in Oleate-Capped CdSe Nanocrystals under High-Intensity Photoexcitation

“…Ligands are critical not only for colloidal stability but also for passivating the undercoordinated surfaces of NCs to promote radiative recombination rather than surface trapping . The coupling of the optoelectronic properties produced by the semiconductor core with robust syntheses and solution processability bolstered by capping ligands have led to semiconductor NC implementation for a wide range of applications including photocatalysis, − photovoltaics, − display technologies, , biosensing, and lasing. , …”

Spectroelectrochemical Characterization of Energetics in Type I vs Quasi-Type II Heterojunctions in CdSe@CdS Nanorod Films

“…Creation of compositional and/or structural asymmetry in semiconductor-based energy conversion systems creates energetic offsets in both conduction and valence band energies ( E CB and E VB , respectively), across heterojunctions, at nanometer length scales. These offsets are often the principal driving force for creation of mobile charge carriers ultimately used in redox reactions to form chemical products such as H 2 . − This asymmetry is often achieved through creation of core–shell constructs for spherical particles (core–shell quantum dots) and more elaborate core–shell constructs including nanorods (NRs) and tetrapod configurations. ,,− II–VI semiconductor materials provide synthetic accessibility and relative ease of tuning spectroscopic, electronic, and catalytic properties via size and shape control of two adjacent semiconductor materials. − ,− ,− Heterostructured nanomaterials based on CdS, CdSe, and CdTe (e.g., CdSe@CdS core–shell QDs, NRs, and tetrapods) provide a wide variety of structural variations in both the core and shell materials, a portfolio of capping ligands that ensure solution processability and impact on intrinsic energetics, and some examples where tethering of these materials to electrode surfaces further impacts energetics and energy conversion efficiencies. − …”