Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis.

Hot-injection synthesis is renowned for producing semiconductor nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic yet experimental evidence supporting the pertinence of these concepts is scant. By monitoring a CdSe synthesis in-situ with X-ray scattering, we find that nucleation is an extended event that coincides with growth during 15-20% of the reaction time. Moreover, we show that size focusing outpaces predictions of diffusion-limited growth. This observation indicates that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations confirm that this so-called superfocusing can lengthen the nucleation period and promote size focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse semiconductor nanocolloids.

Carboxylic-Acid-passivated metal oxide nanocrystals: ligand exchange characteristics of a new binding motif.

Ligand exchange is central in the processing of inorganic nanocrystals (NCs) and requires understanding of surface chemistry. Studying sterically stabilized HfO2 and ZrO2 NCs using (1) H solution NMR and IR spectroscopy as well as elemental analysis, this paper demonstrates the reversible exchange of initial oleic acid ligands for octylamine and self-adsorption of oleic acid at NC surfaces. Both processes are incompatible with an X-type binding motif of carboxylic acids as reported for sulfide and selenide NCs. We argue that this behavior stems from the dissociative adsorption of carboxylic acids at the oxide surface. Both proton and carboxylate moieties must be regarded as X-type ligands yielding a combined X2 binding motif that allows for self-adsorption and exchange for L-type ligands.

The micropatterning of layers of colloidal quantum dots with inorganic ligands using selective wet etching.

The micropatterning of layers of colloidal quantum dots (QDs) stabilized by inorganic ligands is demonstrated using PbS core and CdSe/CdS core/shell QDs. A layer-by-layer approach is used to assemble the QD films, where each cycle involves the deposition of a QD layer by dip-coating, and the replacement of the native organic ligands by inorganic moieties, such as OH(-) and S(2-), followed by a thorough cleaning of the resulting film. This results in a smooth and crack-free QD film on which a photoresist can be spun. The micropatterns are defined by a positive photoresist, followed by the removal of uncovered QDs by selective wet etching with an HCl/H3PO4 mixture. The resulting patterns can have submicron feature dimensions, limited by the resolution of the lithographic process, and can be formed on planar and 3D substrates. It is shown that the photolithography and wet etching steps have little effect on the photoluminescence quantum yield of CdSe/CdS QDs. Compared with the unpatterned CdSe/CdS QD film, only a 10% degradation in the quantum yield is observed. These results demonstrate the feasibility of the proposed micropatterning method to implement the large-scale device integration of colloidal quantum dots.

Multiple dot-in-rod PbS/CdS heterostructures with high photoluminescence quantum yield in the near-infrared.

Pb cations in PbS quantum rods made from CdS quantum rods by successive complete cationic exchange reactions are partially re-exchanged for Cd cations. Using STEM-HAADF, we show that this leads to the formation of unique multiple dot-in-rod PbS/CdS heteronanostructures, with a photoluminescence quantum yield of 45-55%. We argue that the formation of multiple dot-in-rods is related to the initial polycrystallinity of the PbS quantum rods, where each PbS crystallite transforms in a separate PbS/CdS dot-in-dot. Effective mass modeling indicates that electronic coupling between the different PbS conduction band states is feasible for the multiple dot-in-rod geometries obtained, while the hole states remain largely uncoupled.

Langmuir-Blodgett monolayers of colloidal lead chalcogenide quantum dots: morphology and photoluminescence.

Monolayers of PbSe and PbS quantum dots and PbSe/CdSe core/shell quantum dots made by Langmuir-Blodgett deposition are compared, with a focus on the formation, the morphology and the photoluminescence properties of the films. It is shown that PbSe quantum dots suffer from oriented attachment and a complete quenching of their photoluminescence after Langmuir-Blodgett processing. While the oriented attachment can be resolved by growing a CdSe shell around the PbSe core QDs, the photoluminescence of PbSe/CdSe Langmuir-Blodgett monolayers remains largely quenched. In the case of PbS quantum dots, the formation of a close-packed monolayer is more difficult, yet the resulting films show no sign of oriented attachment and their photoluminescence is comparable to that of the original, suspended quantum dots. In spite of their slow oxidation in a matter of weeks, these results mark PbS quantum dots as the preferred material for light-emitting applications in the near IR based on Langmuir-Blodgett monolayers of lead chalcogenide quantum dots.

A phonon scattering bottleneck for carrier cooling in lead chalcogenide nanocrystals.

The cooling dynamics of hot charge carriers in colloidal lead chalcogenide nanocrystals is studied by hyperspectral transient absorption spectroscopy. We demonstrate a transient accumulation of charge carriers at a high energy critical point in the Brillouin zone. Using a theoretical study of the cooling rate in lead chalcogenides, we attribute this slowing down of charge carrier cooling to a phonon scattering bottleneck around this critical point. The relevance of this observation for the possible harvesting of the excess energy of hot carriers by schemes such as multiexciton generation is discussed.

Less is more. Cation exchange and the chemistry of the nanocrystal surface.

We link the extent of Pb for Cd cation exchange reactions in PbS colloidal quantum dots (QDs) to their surface chemistry. Using PbS QDs with either a full or a partial surface coverage by excess Pb, we demonstrate the central role played by vacant cation sites on the QD surface. They facilitate the adsorption of cations from solution, and they act as a source of vacancies needed for the transport of cations through the crystal lattice. This model explains our finding that the cation exchange reaction runs to completion when using a low Cd excess in the exchange bath, while it is impeded by a high Cd excess. Whereas in the latter case, the QD surface is poisoned by surface Cd, the former conditions provide the mixture of surface Cd and vacant surface sites the exchange reaction needs to proceed. This understanding provides a missing link needed to build a unifying mechanistic picture of cation exchange reactions at nanocrystals.

Giant and broad-band absorption enhancement in colloidal quantum dot monolayers through dipolar coupling.

The absorption cross section of colloidal quantum dots in close-packed monolayers shows a 4 (CdSe) to 5-fold (PbS) enhancement compared to quantum dots in a dilute dispersion. Quantitative agreement is demonstrated between the value and the size dependence of the enhancement and theoretical model predictions based on dipolar coupling between neighboring quantum dots. This collective optical behavior offers a new degree of freedom in the custom design of optical properties for electro-optical devices.

Size-tunable, bright, and stable PbS quantum dots: a surface chemistry study.

PbS Qdots are synthesized using PbCl2 and elemental sulfur as precursors. The available size range is significantly expanded using tri-n-octylphosphine (TOP), enabling the synthesis of monodisperse suspensions of Qdots with a mean size varying between 3 and 10 nm. The ligand composition and dynamics are investigated with nuclear magnetic resonance (NMR) spectroscopy. We show that the Qdots are passivated solely by highly dynamic OlAm ligands, even when TOP is employed during synthesis. In this respect, TOP is a compound strongly modifying the Qdot synthesis, without affecting the final Qdot surface chemistry. Next, the OlAm ligands are exchanged for oleic acid (OlAc). NMR data show that the OlAc ligands are tightly bound to the Qdot surface, with a coverage of 3.0±0.4 nm(-2). In addition, we demonstrate that they are bound as oleate ions. Combining this with the inorganic Qdot composition, we observe that charge-neutral Qdots are obtained when taking into account the charge of the stoichiometric PbS Qdot core, the surface excess of Pb ions, the surface-adsorbed Cl ions and the oleate ligands. The Qdot suspensions are stable under atmospheric conditions, showing no changes in the NMR and absorbance spectra for several weeks. Finally, we determine the photoluminescence quantum yield (PL QY) for OlAc-capped PbS Qdots, synthesized either with or without TOP. In both cases, they are highly luminescent, with PL QY values varying between 20 and 90%, depending on the Qdot size.

The different nature of band edge absorption and emission in colloidal PbSe/CdSe core/shell quantum dots.

We present a quantitative analysis of the absorption and luminescence of colloidal PbSe/CdSe core/shell quantum dots (QDs). In absorption, both the energy and the oscillator strength of the first exciton transition coincide with that of plain PbSe QDs. In contrast, luminescence lifetime measurements indicate that the oscillator strength of the emitting transition is reduced by at least a factor of 4 compared to PbSe core QDs. Moreover, the addition of an electron scavenger quenches the PbSe/CdSe emission, while a hole scavenger does not. This implies that the electron wave function reaches the QD surface, while the hole is confined to the PbSe core. These observations are consistent with calculations based on the effective mass model, which show that PbSe/CdSe QDs are at the boundary between the type-I and quasi-type-II regime, where the electron spreads over the entire nanoparticle and the hole remains confined in the PbSe core. However, as this only leads to a minor reduction of the oscillator strength, it follows that the drastic reduction of the oscillator strength in emission cannot be explained in terms of electron delocalization. In combination with the increased Stokes shift for PbSe/CdSe QDs, this indicates that the emission results from lower energy states that are fundamentally different from the absorbing states.