Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution.

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Ligand Steric Profile Tunes the Reactivity of Indium Phosphide Clusters.

Indium phosphide quantum dots have become an industrially relevant material for solid-state lighting and wide color gamut displays. The synthesis of indium phosphide quantum dots from indium carboxylates and tris(trimethylsilyl)phosphine (P(SiMe3)3) is understood to proceed through the formation of magic-sized clusters, with In37P20(O2CR)51 being the key isolable intermediate. The reactivity of the In37P20(O2CR)51 cluster is a vital parameter in controlling the conversion to quantum dots. Herein, we report structural perturbations of In37P20(O2CR)51 clusters induced by tuning the steric properties of a series of substituted phenylacetate ligands. This approach allows for control over reactivity with P(SiMe3)3, where meta-substituents enhance the susceptibility to ligand displacement, and para-substituents hinder phosphine diffusion to the core. Thermolysis studies show that with complete cluster dissolution, steric profile can modulate the nucleation period, resulting in a nanocrystal size dependence on ligand steric profile. The enhanced stability from ligand engineering also allows for the isolation and structural characterization by single-crystal X-ray diffraction of a new III-V magic-sized cluster with the formula In26P13(O2CR)39. This intermediate precedes the In37P20(O2CR)51 cluster on the InP QD reaction coordinate. The physical and electronic structure of this cluster are analyzed, providing new insight into previously unrecognized relationships between II-VI and III-V materials and the discrete growth of III-V cluster intermediates.

Navigating the Potential Energy Surface of CdSe Magic-Sized Clusters: Synthesis and Interconversion of Atomically Precise Nanocrystal Polymorphs.

Magic-sized clusters (MSCs) are kinetically stable, atomically precise intermediates along the quantum dot (QD) reaction potential energy surface. Literature precedent establishes two classes of cadmium selenide MSCs with QD-like inorganic cores: one class is proposed to be cation-rich with a zincblende crystal structure, while the other is proposed to be stoichiometric with a "wurtzite-like" core. However, the wide range of synthetic protocols used to access MSCs has made direct comparisons of their structure and surface chemistry difficult. Furthermore, the physical and chemical relationships between MSC polymorphs are yet to be established. Here, we demonstrate that both cation-rich and stoichiometric CdSe MSCs can be synthesized from identical reagents and can be interconverted through the addition of either excess cadmium or selenium precursor. The structural and compositional differences between these two polymorphs are contrasted using a combination of 1H NMR spectroscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, inductively coupled plasma optical emission spectroscopy, and UV-vis transient absorption spectroscopy. The subsequent polymorph interconversion reactions are monitored by UV-vis absorption spectroscopy, with evidence for an altered cluster atomic structure observed by powder XRD and PDF analysis. This work helps to simplify the complex picture of the CdSe nanocrystal landscape and provides a method to explore structure-property relationships in colloidal semiconductors through atomically precise synthesis.

Colloidal, Room-Temperature Growth of Metal Oxide Shells on InP Quantum Dots.

We demonstrate colloidal, layer-by-layer growth of metal oxide shells on InP quantum dots (QDs) at room temperature. We show with computational modeling that native InP QD surface oxides give rise to nonradiative pathways due to the presence of surface-localized dark states near the band edges. Replacing surface indium with zinc to form a ZnO shell results in reduced nonradiative decay and a density of states at the valence band edge that resembles defect-free, stoichiometric InP. We then developed a synthetic strategy using stoichiometric amounts of common atomic layer deposition precursors in alternating cycles to achieve layer-by-layer growth. Metal-oxide-shelled InP QDs show bulk and local structural perturbations as determined by X-ray diffraction and extended X-ray absorption fine structure spectroscopy. Upon growing ZnSe shells of varying thickness on the oxide-shelled QDs, we observe increased photoluminescence (PL) quantum yields and narrowing of the emission linewidths that we attribute to decreased ion diffusion to the shell, as supported by phosphorus X-ray emission spectroscopy. These results present a versatile strategy to control QD interfaces for novel heterostructure design by leveraging surface oxides. This work also contributes to our understanding of the connections between structural complexity and PL properties in technologically relevant colloidal optoelectronic materials.

Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots.

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it.

Photoinduced Charge Transfer from Quantum Dots Measured by Cyclic Voltammetry.

Measuring and modulating charge-transfer processes at quantum dot interfaces are crucial steps in developing quantum dots as photocatalysts. In this work, cyclic voltammetry under illumination is demonstrated to measure the rate of photoinduced charge transfer from CdS quantum dots by directly probing the changing oxidation states of a library of molecular charge acceptors, including both hole and electron acceptors. The voltammetry data demonstrate the presence of long-lived charge donor states generated by native photodoping of the quantum dots as well as a positive correlation between driving force and rate of charge transfer. Changes to the voltammograms under illumination follow mechanistic predictions from the ErCi' zone diagram, and electrochemical modeling allows for measurement of the rate of productive electron transfer. Observed rates for photoinduced charge transfer are on the order of 0.1 s-1, which are distinct from the picosecond dynamics measured by conventional transient optical spectroscopy methods and are more closely connected to the quantum yield of light-mediated chemical transformations.

CO2 Hydrogenation Catalyzed by a Ruthenium Protic N-Heterocyclic Carbene Complex.

We describe the hydrogenation of CO2 to formate catalyzed by a Ru(II) bis(protic N-heterocyclic carbene, p-NHC) phosphine complex [Ru(bpy)(MeCN)(PPh(p-NHC)2)](PF6)2 (1). Under catalytic conditions (20 µmol catalyst, 20 bar CO2, 60 bar H2, 5 mL THF, 140 °C, 16 h), the activity of 1 is limited only by the amount of K3PO4 present in the reaction, yielding a nearly 1:1 ratio of turnover number (TON) to equivalents of K3PO4 (relative to 1), with the highest TON = 8040. Additionally, analysis of the reaction solution post-run reveals the catalyst intact with no free ligand observed. Stoichiometric studies, including examination of unique carbamate and hydride complexes as relevant intermediates, were carried out to probe the operative mechanism and understand the importance of metal-ligand cooperativity in this system.

Tuning the interfacial stoichiometry of InP core and InP/ZnSe core/shell quantum dots.

We demonstrate fine-tuning of the atomic composition of InP/ZnSe quantum dots (QDs) at the core/shell interface. Specifically, we control the stoichiometry of both anions (P, As, S, and Se) and cations (In and Zn) at the InP/ZnSe core/shell interface and correlate these changes with the resultant steady-state 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 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 that 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 gives rise to electronic defects contributing to the redshift in the absorption. These results highlight a general strategy to atomistically tune the interfacial stoichiometry of InP QDs using surface-limited reaction chemistry allowing for precise correlations with the electronic structure and photophysical properties.

Modeling Equilibrium Binding at Quantum Dot Surfaces Using Cyclic Voltammetry.

Cyclic voltammetry is demonstrated as a useful method to model equilibrium binding between quantum dots and redox active small molecules. Specifically, the interaction of a library of ferrocene derivatives with CdSe quantum dots is examined. For the strongly interacting systems, ferrocene carboxylic acid (FcCOOH) and ferrocene hexanethiol (Fc-hexSH), the binding equilibria can be quantitatively deduced by modeling the cyclic voltammetry data. This modeling allows extraction of the diffusion coefficients, equilibrium constants associated with both the reduced and oxidized species, and forward and reverse rates associated with binding for both the reduced and oxidized species. Taken together these data give direct insight into the binding of small molecules to quantum-dot surfaces as a function of oxidation state, critical information for the design of quantum dots as photoredox catalysts and charge transfer mediators.

Covalently Linked, Two-Dimensional Quantum Dot Assemblies.

Using nanoscale building blocks to construct hierarchical materials is a radical new branch point in materials discovery that promises new structures and emergent functionality. Understanding the design principles that govern nanoparticle assembly is critical to moving this field forward. By exploiting mixed ligand environments to target patchy nanoparticle surfaces, we have demonstrated a novel method of colloidal quantum dot (QD) assembly that gives rise to 2D structures. The equilibration of solutions of spherical and quasispherical QDs, including CdS, CdSe, and InP, with 2,2'-bipyridine-5,5'-diacrylic acid resulted in the preferential formation of 2D assemblies over the course of days as determined by transmission electron microscopy analysis. Small-angle X-ray scattering confirms the existence of the QD assemblies in solution. The dependence of the assembly on linker properties (length and rigidity), linker concentration, and total concentration was investigated, together with the data point to a mechanism involving ligand redistribution to create a patchy surface that maximizes the steric repulsion of neighboring QDs. By operating in an underexchanged regime, the arising patchiness results in enthalpically preferred directions of cross-linking that can be accessed by thermal equilibration.

Hydrogen on Cobalt Phosphide.

Cobalt phosphide (CoP) is one of the most promising earth-abundant replacements for noble metal catalysts for the hydrogen evolution reaction (HER). Critical to HER is the binding of H atoms. While theoretical studies have computed preferred sites and energetics of hydrogen bound to transition metal phosphide surfaces, direct experimental studies are scarce. Herein, we describe measurements of stoichiometry and thermochemistry for hydrogen bound to CoP. We studied both mesoscale CoP particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal CoP nanoparticles. Treatment with H2 introduced large amounts of reactive hydrogen to CoP, ca. 0.2 H per CoP unit, and on the order of one H per Co or P surface atom. This was quantified using alkyne hydrogenation and H-atom transfer reactions with phenoxy radicals. Reactive H atoms were even present on the as-prepared materials. On the basis of the reactivity of CoP with various molecular hydrogen donating and accepting reagents, the distribution of binding free energies for H atoms on CoP was estimated to be roughly 51-66 kcal mol-1 (ΔG°H ≅ 0 to -0.7 eV vs H2). Operando X-ray absorption spectroscopy gave preliminary indications about the structure of hydrogenated CoP, showing a slight lattice expansion and no significant change of the effective nuclear charge of Co under H2-flow. These results provide a new picture of catalytically active CoP, with a substantial amount of reactive H atoms. This is likely of fundamental relevance for its catalytic and electrocatalytic properties. Additionally, the approach developed here provides a roadmap to examine hydrogen on other materials.

Probing the Surface Structure of Semiconductor Nanoparticles by DNP SENS with Dielectric Support Materials.

Surface characterization is crucial for understanding how the atomic-level structure affects the chemical and photophysical properties of semiconducting nanoparticles (NPs). Solid-state nuclear magnetic resonance spectroscopy (NMR) is potentially a powerful technique for the characterization of the surface of NPs, but it is hindered by poor sensitivity. Dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) has previously been demonstrated to enhance the sensitivity of surface-selective solid-state NMR experiments by 1-2 orders of magnitude. Established sample preparations for DNP SENS experiments on NPs require the dilution of the NPs on mesoporous silica. Using hexagonal boron nitride (h-BN) to disperse the NPs doubles DNP enhancements and absolute sensitivity in comparison to standard protocols with mesoporous silica. Alternatively, precipitating the NPs as powders, mixing them with h-BN, and then impregnating the powdered mixture with radical solution leads to further 4-fold sensitivity enhancements by increasing the concentration of NPs in the final sample. This modified procedure provides a factor of 9 improvement in NMR sensitivity in comparison to previously established DNP SENS procedures, enabling challenging homonuclear and heteronuclear 2D NMR experiments on CdS, Si, and Cd3P2 NPs. These experiments allow NMR signals from the surface, subsurface, and core sites to be observed and assigned. For example, we demonstrate the acquisition of DNP-enhanced 2D 113Cd-113Cd correlation NMR experiments on CdS NPs and natural isotropic abundance 2D 13C-29Si HETCOR of functionalized Si NPs. These experiments provide a critical understanding of NP surface structures.

Conversion Reactions of Atomically Precise Semiconductor Clusters.

Clusters are unique molecular species that can be viewed as a bridge between phases of matter and thus between disciplines of chemistry. The structural and compositional complexity observed in cluster chemistry serves as an inspiration to the material science community and motivates our search for new phases of matter. Moreover, the formation of kinetically persistent cluster molecules as intermediates in the nucleation of crystals makes these materials of great interest for determining and controlling mechanisms of crystal growth. Our lab developed a keen interest in clusters insofar as they relate to the nucleation of nanoscale semiconductors and the modeling of postsynthetic reaction chemistry of colloidal materials. In particular, our discovery of a structurally unique In37P20X51 (X = carboxylate) cluster en route to InP quantum dots has catalyzed our interest in all aspects of cluster conversion, including the use of clusters as precursors to larger nanoscale colloids and as platforms for examining postsynthetic reaction chemistry. This Account is presented in four parts. First, we introduce cluster chemistry in a historical context with a focus on main group, metallic, and semiconductor clusters. We put forward the concept of rational, mechanism-driven design of colloidal semiconductor nanocrystals as the primary motivation for the studies we have undertaken. Second, we describe the role of clusters as intermediates both in the synthesis of well-known material phases and in the discovery of unprecedented nanomaterial structures. The primary distinction between these two approaches is one of kinetics; in the case of well-known phases, we are often operating under high-temperature thermolysis conditions, whereas for materials discovery, we are discovering strategies to template the growth of kinetic phases as dictated by the starting cluster structure. Third, we describe reactions of clusters as model systems for their larger nanomaterial progeny with a primary focus on cation exchange. In the case of InP, cation exchange in larger nanostructures has been challenging due to the covalent nature of the crystal lattice. However, in the higher energy, strained cluster intermediates, cation exchange can be accomplished even at room temperature. This opens opportunities for accessing doped and alloyed nanomaterials using postsynthetically modified clusters as single-source precursors. Finally, we present surface chemistry of clusters as the gateway to subsequent chemistry and reactivity, and as an integral component of cluster structure and stability. Taken as a whole, we hope to make a compelling case for using clusters as a platform for mechanistic investigation and materials discovery.

Quantifying Ligand Exchange on InP Using an Atomically Precise Cluster Platform.

The surface chemistry of a colloidal nanoparticle is intrinsic to both its structure and function. It is therefore necessary to characterize the surfaces of colloidal materials to rationally underpin any synthetic, catalytic, or transformative mechanisms they enable. Here we characterize the surface properties of colloidal InP clusters and quantum dots by examining the binding of traditional stabilizing ligands including carboxylates, phosphonates, and thiolates. By using the In37P20X51 (X = carboxylate) cluster species as an ideally monodisperse and well-defined starting scaffold, we quantify surface-exchange equilibria. Using quantitative 1H and 31P NMR spectroscopy, we show that 1:1 metathesis-type binding models are insufficient to fully describe the surface dynamics. In particular, for the case of the highly reversible carboxylate ligand exchange, a more detailed isotherm approach using a two-site, competitive model is necessary. This model is used to deconvolute L- and X-type binding modalities. We additionally quantify the reversible and irreversible ligand-exchange reactions observed in the thiolate and phosphonate  systems.

Conversion of InP Clusters to Quantum Dots.

Understanding and deconvoluting the different mechanisms involved in the synthesis of nanomaterials is necessary to make uniform materials with desirable function. In this study, in situ spectroscopic methods were used to study exchange reactions at the surface of indium phosphide clusters, revealing that the cluster surface lacks significant dynamics on the NMR time-scale at room temperature. The exchange of surface carboxylate ligands can be induced at elevated temperatures and with the addition of carboxylic acid and indium carboxylate. These studies suggest that carboxylate may be a key ingredient in promoting cluster dissolution to larger nanostructures. Toward this end, the evolution of InP clusters was examined by in situ UV-vis spectroscopy, revealing cluster dissolution and renucleation that is dramatically dependent on the concentration of carboxylate. In addition to the concentration of exogenous ligands, the rate of particle growth and final product distribution were dependent on temperature and initial cluster concentration. These results, taken together, suggest a mechanism of cluster evolution involving cluster dissociation to form multiple reactive monomer species that renucleate and grow to larger nanomaterials. Nonproductive monomer degradation is observed in the lower temperature regime (<200 °C), suggesting a critical temperature threshold for efficient cluster to quantum dot conversion.

Effects of Zn2+ and Ga3+ doping on the quantum yield of cluster-derived InP quantum dots.

As the commercial display market grows, the demand for low-toxicity, highly emissive, and size-tunable semiconducting nanoparticles has increased. Indium phosphide quantum dots represent a promising solution to these challenges; unfortunately, they typically suffer from low inherent emissivity resulting from charge carrier trapping. Strategies to improve the emissive characteristics of indium phosphide often involve zinc incorporation into or onto the core itself and the fabrication of core/shell heterostructures. InP clusters are high fidelity platforms for studying processes such as cation exchange and surface doping with exogenous ions since these clusters are used as single-source precursors for quantum dot synthesis. Here, we examined the incorporation of zinc and gallium ions in InP clusters and the use of the resultant doped clusters as single-source precursors to emissive heterostructured nanoparticles. Zinc ions were observed to readily react with InP clusters, resulting in partial cation exchange, whereas gallium resisted cluster incorporation. Zinc-doped clusters effectively converted to emissive nanoparticles, with quantum yields strongly correlated with zinc content. On the other hand, gallium-doped clusters failed to demonstrate improvements in quantum dot emission. These results indicate stark differences in the mechanisms associated with aliovalent and isovalent doping and provide insight into the use of doped clusters to make emissive quantum dots.

Improved HER Catalysis through Facile, Aqueous Electrochemical Activation of Nanoscale WSe2.

In the search for nonprecious metal catalysts for the hydrogen evolution reaction (HER), transition metal dichalcogenides (TMDCs) have been proposed as promising candidates. Here, we present a facile method for significantly decreasing the overpotential required for catalyzing the HER with colloidally synthesized WSe2. Solution phase deposition of 2H WSe2 nanoflowers (NFs) onto carbon fiber electrodes results in low catalytic activity in 0.5 M H2SO4 with an overpotential at -10 mA/cm2 of greater than 600 mV. However, two postdeposition electrode processing steps significantly reduce the overpotential. First, a room-temperature treatment of the prepared electrodes with a dilute solution of the alkylating agent Meerwein's salt ([Et3O][BF4]) results in a reduction in overpotential by approximately 130 mV at -10 mA/cm2. Second, we observe a decrease in overpotential of approximately 200-300 mV when the TMDC electrode is exposed to H+, Li+, Na+, or K+ ions under a reducing potential. The combined effect of ligand removal and electrochemical activation results in an improvement in overpotential by as much as 400 mV. Notably, the Li+ activated WSe2 NF deposited carbon fiber electrode requires an overpotential of only 243 mV to generate a current density of -10 mA/cm2. Measurement of changes in the material work function and charge transfer resistance ultimately provide rationale for the catalytic improvement.

Deterministic Positioning of Colloidal Quantum Dots on Silicon Nitride Nanobeam Cavities.

Engineering an array of precisely located cavity-coupled active media poses a major experimental challenge in the field of hybrid integrated photonics. We deterministically position solution-processed colloidal quantum dots (QDs) on high quality (Q)-factor silicon nitride nanobeam cavities and demonstrate light-matter coupling. By lithographically defining a window on top of an encapsulated cavity that is cladded in a polymer resist, and spin coating the QD solution, we can precisely control the placement of the QDs, which subsequently couple to the cavity. We show rudimentary control of the number of QDs coupled to the cavity by modifying the size of the window. Furthermore, we demonstrate Purcell enhancement and saturable photoluminescence in this QD-cavity platform. Finally, we deterministically position QDs on a photonic molecule and observe QD-coupled cavity supermodes. Our results pave the way for precisely controlling the number of QDs coupled to a cavity by engineering the window size, the QD dimension, and the solution chemistry and will allow advanced studies in cavity enhanced single photon emission, ultralow power nonlinear optics, and quantum many-body simulations with interacting photons.

Templated Growth of InP Nanocrystals with a Polytwistane Structure.

We have synthesized InP nanocrystals of an unprecedented crystal phase at low temperature (35-100 °C) by templated growth of InP magic-sized clusters. With the addition of stoichiometric equivalents of P(SiMe3 )3 to the starting cluster, we demonstrate nanocrystal growth mediated through a partial dissolution and recrystallization pathway. This growth process was monitored using a combination of in situ UV/Vis and 31 P NMR spectroscopy, revealing the intermediacy of smaller cluster species of higher symmetry. The nanocrystals that result from this templated growth exhibit a crystal structure that is neither zincblende nor wurtzite, and instead is derived from the original cluster. This structure is best described as a 3D polytwistane phase as deduced from a combination of X-ray diffraction, Raman, and solid-state NMR spectroscopy methods.