Gordana Dukovic

Photochemical reduction reactions catalyzed by nanocrystals (NCs) and enzymes require both electron transfer (ET) and hole transfer (HT) reactions to occur in spatial and temporal proximity. As a result, the observed photochemical reaction rates are complex functions of many competing ET and HT reactions. To facilitate interpretation of complex photochemical datasets, we developed a model that simultaneously evaluates the rates of both the reduction and oxidation reactions. We applied this model to electron-intensive reduction of dinitrogen to ammonia by CdS NCs and nitrogenase MoFe protein. A fit of the model to the kinetics of product accumulation determined that during prolonged illuminations the efficiency of N2 reduction was limited by MoFe protein oxidation, facilitated by the accumulation of the products of the oxidation reaction. Modeling time-resolved populations of catalytic intermediates revealed variability in the rates of reduction, indicating that accumulation of hydrides and N2 activation influenced the rates of electron transfer.

We explore silane-coated formamidinium lead bromide (FAPbBr₃) quantum dots (QDs) as single photon emitters and compare them to FAPbBr₃ QDs passivated with a phosphoethylammonium derivative (PEAC₈C₁₂), which represents current state-of-the-art ligand passivation. We compare properties including single-photon purity (g⁽²⁾(τ)), line width, blinking, and photostability. We find that at room temperature, these silane-coated dots perform comparably to PEAC₈C₁₂-passivated dots, while exhibiting improvements in photostability. However, we find that at 4 K, silane-coated FAPbBr₃ QDs perform worse than the PEAC₈C₁₂-passivated samples, exhibiting faster blue-shifting and photobleaching under illumination. Analysis of fluorescence lifetime intensity distributions from the photon-counting data indicates increased efficiency of fast nonradiative processes in the silane-coated QDs at 4 K. We propose a trion-related degradation pathway at low temperatures that is consistent with the observed kinetics and estimate that at 4 K with 6.1 μJ/cm², 472 nm excitation the silane-coated QDs build up double the trion population of their PEAC₈C₁₂-passivated counterparts.

Developing systems that can efficiently capture photon energy and convert this energy into fuels and chemicals requires understanding how to assemble molecular components with diverse functions into complete systems possessing selectivity and efficiency in directing charge carriers to catalytic reactions. There are many challenges to achieving this goal. One promising approach is the development of hybrid systems that combine semiconductor nanocrystals (NCs) for light capture and enzymes as efficient catalysts. Such biohybrid systems capitalize on the tunable electronic and optical properties of NCs while leveraging the unmatched specificity and efficiency of enzymes in catalyzing chemical reactions, thereby offering opportunities to surpass the limitations of each component alone. Here, we focus on recent progress in developing a biohybrid system that combines CdS NCs for photon capture with the enzyme nitrogenase to accomplish light-driven dinitrogen (N₂) reduction to ammonia (NH₃). Integrating light-harvesting materials with biological catalysts requires a deep understanding of NC properties, protein stability, and electron transfer (ET), making it an inherently multidisciplinary problem. The reduction of N₂ to NH₃ is a challenging reaction, with a high demand in both agriculture and industrial chemical production. This reaction is intrinsically energy intensive, due to the need to activate the N≡N triple bond. The current standard industrial approach to N₂ reduction, the Haber–Bosch reaction, obtains the necessary energy input from fossil fuels, whereas biological systems capable of N₂ reduction utilize the hydrolysis of ATP as their energy source. Replacing these costly, energy-intensive inputs with renewable light energy represents a critical step toward sustainable NH₃ production. Recent progress has demonstrated that semiconductor CdS NCs can be coupled to the catalytic component of nitrogenase, the MoFe protein, to form a biohybrid CdS NC:MoFe protein complex, enabling light-driven N₂ reduction rather than energy input from fossil fuels or ATP. This illustrates how inorganic NCs can functionally replace the natural Fe protein partner, yielding a biohybrid catalyst that enables controlled electron delivery and provides not only light-driven NH₃ production but also new approaches for probing enzyme catalytic function. The CdS NC:MoFe protein biohybrid system enables light-initiated electron delivery at ambient temperature, as well as temperatures below freezing, allowing for stabilization and spectroscopic characterization of key reaction intermediates. These findings highlight how photochemical biohybrids can serve as both functional catalysts and mechanistic probes. Beyond studies of the nitrogenase mechanism, studies of the CdS NC:MoFe system reveal how variables such as NC size, electrostatic binding interactions, and sacrificial electron donors (SEDs) govern complex stability, charge transfer efficiency, and catalytic performance. In addition, studies of nitrogenase and the high activation barrier for N₂ reduction are enabling investigations of new and interesting questions regarding the properties and limitations of NC biocatalysis. In this Account, we describe the key features of CdS NC:MoFe protein biohybrids and the parameters for optimal light-driven N₂ reduction, and how controlling ET with light illuminates the path to new insights into the nitrogenase mechanism.

Cation exchange in molten salts has emerged as a method to synthesize colloidal III-V quantum dots with tunable composition and excellent optoelectronic properties. However, the atomistic details of how cation exchange occurs and the elemental distribution in the products remain elusive. In this work, we use scanning transmission electron microscopy to reveal the chemical and structural evolution of In_1-xGa_xP_1-yAs_y nanocrystals made by cation exchange of InP_1-yAs_y nanocrystals in a gallium-containing molten salt. The starting material and In_1-xGa_xP_1-yAs_y nanocrystals at multiple time points in the reaction were characterized by aberration-corrected HAADF-STEM and STEM-EDS. HAADF-STEM reveals subtle changes in size and shape, but the tetrahedral morphology and zinc-blende crystal structure are maintained throughout the reaction. We overcome the technical challenges of beam damage and low signals in STEM-EDS through a combination of experimental optimization and image processing to yield elemental composition maps representative of individual nanocrystals. This analysis revealed that the particles have Ga-rich surfaces at the early reaction stage. As the reaction proceeds, Ga permeates the lattice, but the elemental distribution remains graded with In enrichment in the core even after 16 h. To investigate the origin of the observed elemental distributions, we use finite element analysis to model the cation exchange as a diffusion-limited process, which suggests that cation exchange becomes progressively more difficult as In atoms are replaced by Ga. We discuss the implication of the graded composition on optical properties. More broadly, the image processing techniques developed here could be applied to other compositionally complex nanocrystals.

Hybrid nanocomposites including quantum dots (QDs) and molecular dyes offer tunability of charge and energy transfer processes, which are attractive for applications spanning from light-emitting devices to photocatalysis and photon upconversion. Core@shell QDs have garnered attention for their ability to modulate charge and energy transfer from the core to the dye via the shell. Recent studies suggested that the core@shell QDs are less demanding in terms of the QD-dye electronic coupling requirements and that the distance dependence of transfer efficiencies may vary from what is traditionally observed in molecular systems. However, these studies remain only a few, and the mechanistic role of the shell is often unclear. Here, we synthesize QD-dye nanocomposites including CdSe@AlOx core@shell QDs, and we report the observation of a nearly distance-independent triplet energy transfer (TEnT) to polyaromatic hydrocarbon (PAH) dyes from the CdSe core of up to almost 2 nm. The use of colloidal atomic layer deposition (c-ALD) for the growth of the metal oxide shell enables tunability of the system, which is crucial to elucidate the role of the shell and the thickness dependence of the TEnT. We propose a plausible mechanism where a hole-transfer-mediated TEnT takes place and defects in the metal oxide act as intermediate states, which enable a long-range, distance-independent TEnT. The versatility of the c-ALD methodology, along with the ability of oxides to preserve the optoelectronic properties of QDs, showcases the potential of oxide shells to optimize the QD-dye interaction to achieve long-range TEnT to molecular dyes, opening new avenues for applying QDs as sensitizers in light-harvesting, emission, and conversion applications.