Quantum dots are used in the research laboratory and in commercial applications for their bright, size-tunable luminescence. While empirical synthesis and processing optimization have led to many quantum dot systems with photoluminescence quantum yields at or approaching 100%, our understanding of the chemical principles that underlie this performance and our ability to access such materials on demand have lagged. In this Perspective, we present the status of our understanding of the connections between surface chemistry and quantum dot luminescence. We follow the historical arc that began with shell growth, which then led to an atomistic description of surface-derived charge trapping, and finally has brought us to a more nuanced picture of the role of surface chemistry in luminescence properties, including emerging concepts like surface dipoles and vibronic coupling.

Aminophosphine precursors were used to synthesize copper-doped indium phosphide nanocrystals (InP NCs) via direct doping in a slow-injection bottom-up method and postsynthetic cation exchange. By both methods, the amount of copper incorporated into the NCs could be tuned simply by varying the molar ratio during synthesis. Common postsynthetic surface modifications such as Lewis acid treatment and zinc chalcogenide shelling were performed on these samples, resulting in an enhancement of the copper-based emission from 10% to 40%. For samples with thick shells, the copper-based photoluminescence quantum yield reached over 60%, a record value for doped InP NCs. Time-resolved photoluminescence spectroscopy showed increasing carrier lifetimes after surface treatments concurrent with the disappearance of a 2 ns decay process previously attributed to surface trapping in native InP NCs, showing the broad applicability and consistent impacts of the surface treatments. In this way, we have successfully developed a route to obtain high-quality near-infrared emitters utilizing less toxic alternatives to the popular lead-and cadmium-containing materials.

We demonstrate fine-tuning of the atomic composition of InP/ZnSe QDs at the core/shell interface. Specifically, we control the stoichiometry of both anions (P, As, S, and Se) and cations (In, Zn) at the InP/ZnSe core/shell interface and correlate these changes with the resultant steadystate and time-resolved optical properties of the nanocrystals. The use of reactive trimethylsilyl reagents results in surface-limited reactions that shift the nanocrystal stoichiometry to anion-rich and improve epitaxial growth of the shell layer. In general, anion deposition on the InP QD surface results in a redshift in the absorption, quenching of the excitonic photoluminescence, and a relative increase in the intensity of the broad trap-based photoluminescence, consistent with delocalization of the exciton wavefunction and relaxation of exciton confinement. Time-resolved photoluminescence data for the resulting InP/ZnSe QDs show an overall small change in the decay dynamics on the ns timescale, suggesting the relatively low photoluminescence quantum yields may be attributed to the creation of new thermally activated charge trap states and likely a dark population that is inseparable from the emissive QDs. Cluster-model density functional theory calculations show that the presence of core/shell interface anions give rise to electronic defects contributing to the redshift in the absorption. These results highlight a general strategy to

Semiconducting and metallic metal phosphide nanocrystals have gained increased attention in the materials science and engineering community due to their demonstrated and theoretical promise in both emissive and catalytic applications. Central to realizing the full potential of nanoscale metal phosphides is a thorough understanding of their surfaces and how surface chemistry impacts their function. In this review, we document what is known about the surface chemistry of metal phosphide nanocrystals, including both as synthesized and postsynthetically modified species, and draw a connection between surface chemistry and functional properties. This survey is intended to provide a comprehensive view of metal phosphide nanocrystal surface chemistry and how it differs across the families of phosphide materials. A clear distinction emerges between the semiconducting and metallic phosphides from both a synthetic and applied standpoint. We seek to expose key knowledge gaps and targets for further scientific and technological development. Expected final online publication date for the Annual Review of Materials Science, Volume 51 is August 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Quantum mechanics is a notoriously difficult subject to learn, due to a lack of real-world analogies that might help provide an intuitive grasp of the underlying ideas. Discrete energy levels and absorption and emission wavelengths in atoms are sometimes described as uniquely quantum phenomena, but are actually general to spatially confined waves of any sort. Here, we provide an experiment demonstrating the acoustic spectroscopy of a drum. We show that a struck drum emits sounds of discrete frequencies (tones), and that the lifetime of each emitted tone is directly related by Heisenberg's uncertainty principle to its line width in the drum's frequency spectrum. We also show that a still drum absorbs only those same frequencies when exposed to monochromatic sound from an audio speaker. The resonant motion of the drum membrane is too fast to see by eye (>60 Hz), but can be observed with the aid of a strobe light. The observed resonant modes of the drum are the eigenfunctions of a particle trapped in an infinite circular well, and analogous to the shapes of atomic orbitals (1s, 2p, 3d, 2s, etc.). This experiment can be built for under $50, and is an excellent demo or lab exercise for general and physical chemistry courses to provide a visual example of abstract quantum mechanical ideas.