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The atomic precision of magic-sized clusters offers a route toward narrow emission by eliminating heterogeneous broadening. Herein, we report ultranarrow 467 nm blue emission from cadmium phosphide clusters with a 96 meV line width and as high as 26% photoluminescence quantum yield (PLQY) enabled by tightly bound, bidentate phosphinate ligands. They are obtained through postsynthetic ligand exchange from oleate-capped clusters. The phosphinate maintains the bidentate coordination motif, which does not disturb the metastability of the material but does induce a change in the surface dipole, causing a bathochromic shift in the emission from 457 to 467 nm, which is an optimal wavelength for blue emission. We find that the structure of the ligand tail can heavily influence PLQY and other aspects of the charge carrier dynamics. The ligand exchange protocol can be applied to the related cadmium arsenide clusters, resulting in a narrow 550 nm green emission with a 9% PLQY. 

Semiconductor nanocrystals (NCs) offer prospective use as active optical elements in photovoltaics, light-emitting diodes, lasers, and photocatalysts due to their tunable optical absorption and emission properties, high stability, and scalable solution processing, as well as compatibility with additive manufacturing routes. Over the course of experiments, during device fabrication, or while in use commercially, these materials are often subjected to intense or prolonged electronic excitation and high carrier densities. The influence of such conditions on ligand integrity and binding remains underexplored. Here, we expose CdSe NCs to laser excitation and monitor changes in oleate that is covalently attached to the NC surface using nuclear magnetic resonance as a function of time and laser intensity. Higher photon doses cause increased rates of ligand loss from the particles, with upward of 50% total ligand desorption measured for the longest, most intense excitation. Surprisingly, for a range of excitation intensities, fragmentation of the oleate is detected and occurs concomitantly with formation of aldehydes, terminal alkenes, H2, and water. After illumination, NC size, shape, and bandgap remain constant although low-energy absorption features (Urbach tails) develop in some samples, indicating formation of substantial trap states. The observed reaction chemistry, which here occurs with low photon to chemical conversion efficiency, suggests that ligand reactivity may require examination for improved NC dispersion stability but can also be manipulated to yield desired photocatalytically accessed chemical species. 

The photothermal properties of metal nitrides have recently received significant attention owing to diverse applications in solar energy conversion, photothermal therapies, photoreactions, and thermochromic windows. Here, the photothermal response of titanium nitride nanoparticles is examined using transient X-ray diffraction, in which optical excitation is synchronized with X-ray pulses to characterize dynamic changes in the TiN lattice. Photoinduced diffraction data is quantitatively analyzed to determine increases in the TiN lattice spacing, which are furthermore calibrated against static, temperature-dependent diffraction patterns of the same samples. Measurements of 20 nm and 50 nm diameter TiN nanoparticles reveal transient lattice heating from room temperature up to ∼175 °C for the highest pump fluences investigated here. Increasing excitation intensity drives sublinear increases in lattice temperature, due to increased heat capacity at the higher effective temperatures achieved at higher powers. Temporal dynamics show that higher excitation intensity drives not only higher lattice temperatures, but also unexpectedly slower cooling of the TiN nanoparticles, which is attributed to heating of the solvent proximal to the nanoparticle surface.