Brandi Michelle Cossairt

Dense assemblies of metal nanoparticles offer a scalable route to high-index optical metamaterials, yet their collective response and loss mechanisms remain poorly understood. We synthesize quasi-spherical Ag nanoparticles with diameters ranging from 17 to 66 nm and assemble them into closely-packed monolayers. These films exhibit size-dependent refractive indices reaching a maximum of 10.3 with a low loss region emerging on the red shoulder of the resonance. We introduce a compact analytical model that quantitatively reproduces the measured optical constants and identifies the interparticle gap as the lever controlling the achievable index and loss. We compute Bergman-Milton fundamental limits, finding that our best assemblies reach two-thirds of the high-index–low-loss Pareto frontier for silver/polystyrene composites of arbitrary microstructure with the remaining samples approaching within a factor of 1.7–4. Together, experiment and theory establish silver nanoparticle assemblies as a tunable platform approaching fundamental bounds and define pathways for further performance gains.

Random materials libraries and automated STEM enable accelerated screening and characterization across high-dimensional compositional spaces.

We report a nanocluster-mediated strategy for synthesizing compositionally tunable InAs _x P _{1− x} alloy nanocrystals.

Conspectus: Colloidal nanocrystals exhibit quantum-mechanical phenomena inaccessible in bulk matter, and their properties can be tuned via chemically manipulating their composition, size, shape, and surface chemistry. Decades of development have produced a remarkable synthetic library spanning quantum dots, metal and metal oxide nanocrystals, halide perovskites, and complex multicomponent heterostructures. Yet, the field faces a persistent tension between the sophistication of what can be made and the rigor with which it is reported. Batch-to-batch variability, incomplete characterization, and poorly documented protocols challenge reproducibility and slow the translation of laboratory discoveries into reliable technologies or emerging autonomous workflows. This Tutorial offers guidance across four themes that highlight best practices in colloidal nanochemistry with the aim of fostering rigorous reporting in the field. The first theme concerns synthesis. Every nanocrystal synthesis is, at its most fundamental level, a set of chemical reactions and, therefore, should be communicated with explicit stoichiometries, including by-products where known, and with experimental sections detailed enough for an independent researcher to reproduce the outcome. Attention to hidden variables such as heating rates, reagent purity, atmosphere, and order of addition is essential, and we provide practical guidance on identifying and reporting them. The second theme is surface chemistry, where the influence of ligands on nanocrystal stability, functional properties, and device performance is profound. We discuss ligand coordination across nanocrystal families, from II–VI quantum dots to lead halide perovskites. We also describe how complementary techniques, such as infrared and nuclear magnetic resonance spectroscopy, thermogravimetric analysis, isothermal titration calorimetry, and elemental analysis, can together provide a quantitative picture of the ligand shell. The third theme addresses ex situ characterization, with an emphasis on the minimum suite of measurements needed to unambiguously establish nanocrystal identity. We discuss transmission electron microscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy as complementary and mutually reinforcing tools while highlighting the statistical and analytical rigor required to extract meaningful information from each. Finally, we discuss in situ and advanced characterization as the bridge between empirical synthesis and mechanistic understanding. Real-time probes, such as synchrotron X-ray scattering and absorption, liquid-cell transmission electron microscopy, and time-resolved optical spectroscopy, are increasingly accessible to the community via user facilities. While not required in every study, these advanced tools have fundamentally reshaped our understanding of nucleation, growth, and stability. Thus, we highlight the importance of chemical understanding developed through in situ characterization for establishing a more rational synthesis design that bypasses the current trial-and-error approach. We close by looking toward the future where synthesis sensitivity maps, artificial intelligence, and automation become partners in accelerating discovery, open data practices, and the scientific rigor that must underpin all of it. Good (nano)science today depends on the same foundation it always has, including rigorous, thoroughly documented experiments with an eye towards the future, where advanced characterization and data science are expected to play an increasingly important role.

Magic-sized clusters are atomically precise nanoparticles that eliminate heterogeneous broadening, thereby enabling narrow ensemble emission. However, the discrete structural stability that causes only certain-sized clusters to exist also inherently disallows continuous optoelectronic tunability. New strategies are required to allow magic-sized clusters to continuously cover regions of the visible spectrum. Herein, we investigate the atomic structures of two oleate-ligated Cd3P2-450 and Cd3As2-525 clusters and conclude that they are structurally homologous based on pair distribution function analysis. The structural similarities between Cd3P2-450 and Cd3As2-525 allow alloying of P and As in these magic-sized clusters without perturbing the structure or the impressive optical properties. Ligand exchange for phosphinate then produces Cd3P2–xAsx magic-sized clusters with continuously tunable, narrow, and bright emission from blue (λmax = 469 nm) to green (λmax = 550 nm).

Chalcogenide spinels exhibit electronic and magnetic properties that are tunable through lattice occupancy and composition. Here, we report the first synthesis of spinel-phase Ag2Cr2Se4 and AgCr2Se4 and demonstrate an oxidation-induced ferro-to-ferrimagnetic transition accompanied by a massive increase in TC of >250 K associated with changes in Ag+ site occupancy. The Ag spinels are synthesized through air-free cation exchange of CuCr2Se4 nanocrystals driven by excess AgNO3 to yield ferromagnetic Ag2Cr2Se4 (a = 10.838(4) Å, TC = 152 K, 2 K saturation = 3.1 μB/Cr) with Ag+ occupying both Td 8a and Oh 16c sites. Upon air exposure, Ag2Cr2Se4 converts to ferrimagnetic AgCr2Se4 (a = 10.6085(19) Å, TC > 400 K, 2 K saturation = 2.2 μB/Cr) through a loss of Oh-Ag+ alongside the formation of Ag2Se. We attribute the ferro-to-ferrimagnetic transition to the generation of holes in Ag–Se bands, which couple antiferromagnetically to Cr3+. This Ag–Cr–Se spinel system demonstrates the largest change in TC reported and provides an excellent framework for further exploration of magnetic coupling as a function of oxidation and structure.

Dense assemblies of metal nanoparticles offer a scalable route to high‑index optical metamaterials, yet their collective response and loss mechanisms remain poorly understood. Here, we synthesize quasi‑spherical Ag nanoparticles with diameters ranging from 17 to 66 nm and assemble them into close‑packed monolayers with controlled interparticle gaps. These films exhibit size‑dependent refractive indices ranging from 4.4 to 10.3, with high index and low loss emerging on the red shoulder of the coupled dipolar resonance. We introduce a compact analytical model in which the collective mode is an LC‑type oscillation governed by gap capacitance, f‑sum‑rule‑limited oscillator strength, three loss channels, and polydispersity broadening. The model quantitatively reproduces the measured optical constants and identifies the interparticle gap as the dominant lever controlling the achievable index and loss figure of merit. We compute Bergman-Milton fundamental limits, finding that our best assemblies reach 72% of a high-index–low-loss Pareto frontier for silver/air composites, with the remaining samples approaching within a factor of 2–4. Together, experiment and theory establish silver nanoparticle assemblies as a tunable platform approaching fundamental bounds and define clear pathways for further performance gains.

Surface ligand vibrations govern charge delocalization and nonradiative relaxation in colloidal quantum dots, yet direct experimental access to ligand–exciton coupling remains elusive. In this study, we use polarization-selective two-dimensional electronic–vibrational spectroscopy (2D EV) to resolve coupled electronic and vibrational (vibronic) degrees of freedom in oleate-capped zincblende cadmium sulfide quantum dots. The 2D EV spectra map the vibronic couplings between the carboxylate stretching vibrations (1400–1600 cm–1) and nearly degenerate bright and dark excitonic fine structure states separated by 45 meV. The measurements reveal clear vibrational-coordinate dependence on the exciton resonance frequency, distinct polarization anisotropies between the coupled vibrations and excitonic states, and vibrational mode-specific vibronic coupling strengths that vary with different ligand binding geometries. Complementary electronic structure calculations on Cd34S34(O2CCH3)22H22 nanoclusters indicate substantial ligand participation in band-edge electronic excitations. These results establish 2D EV as a powerful method for interrogating coupled ligand–exciton degrees of freedom in complex nanomaterials.

In recent years, quantum dot light-emitting diodes (QLEDs) have achieved record-high performance, driven by rapid advances in active materials and device engineering. However, fabrication remains a significant barrier for new researchers entering the field, and variability in measurement techniques complicates direct comparison of device performance. Here, we establish a standardized and reproducible protocol for QLED fabrication and characterization, designed to lower the technical threshold, improve experimental reliability, and ensure accurate performance assessment. The protocol was independently validated by researchers with different levels of experience. In addition, we provide detailed troubleshooting guidelines and identify key sources of variability in QLED devices. By integrating electron transport material synthesis, optimized thin-film processing, and standardized device characterization procedures, this protocol ensures consistent device performance and promotes broader adoption of QLED technology across the research community.

Magic-sized clusters are atomically precise nanoparticles that eliminate heterogeneous broadening, thereby enabling narrow ensemble emission. However, the discrete structural stability that causes only certain sized clusters to exist also inherently disallows continuous optoelectronic tunability. New strategies are required to allow magic-sized clusters to continuously cover regions of the visible spectrum. Herein, we investigate the atomic structures of two oleate-ligated Cd3P2-450 and Cd3As2-525 clusters and conclude that they are structurally homologous based on pair distribution function analysis. The structural similarities between Cd3P2-450 and Cd3As2-525 allow alloying of P and As in these magic-sized clusters without perturbing the structure or the impressive optical properties. Ligand exchange for phosphinate then produces Cd3P2-xAsx magic-sized clusters with continuously tunable, narrow, and bright emission from blue (λmax = 469 nm) to green (λmax = 550 nm).