Semiconductor nanocrystals exhibit attractive photophysical properties for use in a variety of applications. Advancing the efficiency of nanocrystal-based devices requires a deep understanding of the physical defects and electronic states that trap charge carriers. Many of these states reside at the nanocrystal surface, which acts as an interface between the semiconductor lattice and the molecular capping ligands. While a detailed structural and electronic understanding of the surface is required to optimize nanocrystal properties, these materials are at a technical disadvantage: unlike molecular structures, semiconductor nanocrystals lack a specific chemical formula and generally must be characterized as heterogeneous ensembles. Therefore, in order for the field to improve current nanocrystal-based technologies, a creative approach to gaining a “molecular-level” picture of nanocrystal surfaces is required. To this end, an expansive toolbox of experimental and computational techniques has emerged in recent years. In this Perspective, we critically evaluate the insight into surface structure and reactivity that can be gained from each of these techniques and demonstrate how their strategic combination is already advancing our molecular-level understanding of nanocrystal surface chemistry.

Quantum dot surfaces are redox active and are known to influence the electronic properties of nanocrystals, yet the molecular-level changes in surface chemistry that occur upon addition of charge are not well understood. In this paper, we report a systematic study monitoring changes in surface coordination chemistry in 3.4 nm CdSe quantum dots upon remote chemical doping by the radical anion reductant sodium naphthalenide (Na­[C10H8]). These studies reveal a new mechanism for charge-balancing the added electrons that localize on surface states through loss of up to ca. 5% of the native anionic carboxylate ligands, as quantified through a combination of UV–vis absorption, 1H NMR, and FTIR spectroscopies. A new method for distinguishing between reduction of surface metal and chalcogenide ions by monitoring ligand loss and optical changes upon doping is introduced. This work emphasizes the importance of studying changes in surface chemistry with remote chemical doping and is more broadly contextualized within the redox reactivity of the QD surface.

A series of Fe(III) complexes were recently reported that are stable and active electrocatalysts for reducing protons into hydrogen gas. Herein, we report the incorporation of these electrocatalysts into a photocatalytic system for hydrogen production. Hydrogen evolution is observed when these catalysts are paired with fluorescein (chromophore) and triethylamine (sacrificial electron source) in a 1:1 ethanol:water mixture. The photocatalytic system is highly active and stable, achieving TONs > 2100 (with respect to catalyst) after 24 h. Catalysis proceeds through a reductive quenching pathway with a quantum yield of over 3%.

Defects arising on the surfaces of semiconductor quantum dots (QDs) limit the applications of these otherwise promising materials. Efforts to rationally passivate these sites using chemical methods, however, are limited by a lack of molecular-level understanding of surface defects. Herein, we report the application of redoxactive chemical probes (E • ′ = −0.48 to −1.9 V vs Fc +/0 ) coupled with spectroscopic tools (nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and UV−vis−NIR) to gain insight into the molecular-level nature and reactivity of defects at PbS QD surfaces. First, Pb ion-based traps coordinated by oleate ligands are studied by reaction with outer-sphere reductants, wherein reduction of a subpopulation of Pb 2+ ions promotes ligand displacement. We observe a correlation between this reactivity and QD size, wherein minimal ligand displacement occurs in small QDs (2.6 nm) but up to ca. 15% of ligands are displaced with larger QDs (>4 nm). The strength of the reductant also has a significant impact; with QD size held constant, more potent reductants induce a higher extent of ligand displacement than mild reductants. Finally, chalcogenide-based defects (disulfides) are interrogated with selective trialkylphosphine reagents. Comparison of QD reactivity with phosphine probes reveals that large PbS QDs possess a greater proportion of native disulfide defects than small QDs. Collectively, this work yields insight into the identities, likely structural environments and reduction potentials of targeted defect sites, thus providing a detailed pictureand roadmap for passivationof common QD surface defects.