Synthesis of PbS Nanorods and Other Ionic Nanocrystals of Complex Morphology by Sequential Cation Exchange Reactions (original) (raw)
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Anisotropic Cation Exchange in PbSe/CdSe Core/Shell Nanocrystals of Different Geometry
Chemistry of Materials, 2012
We present a study of Cd 2+ -for-Pb 2+ exchange in PbSe nanocrystals (NCs) with cube, star, and rod shapes. Prolonged temperature-activated cation exchange results in PbSe/CdSe heterostructured nanocrystals (HNCs) that preserve their specific overall shape, whereas the PbSe core is strongly faceted with dominance of {111} facets. Hence, cation exchange proceeds while the Se anion lattice is preserved, and well-defined {111}/ {111} PbSe/CdSe interfaces develop. Interestingly, by quenching the reaction at different stages of the cation exchange new structures have been isolated, such as core−shell nanorods, CdSe rods that contain one or two separated PbSe dots and fully zinc blende CdSe nanorods. The crystallographically anisotropic cation exchange has been characterized by a combined HRTEM/HAADF-STEM study of heterointerface evolution over reaction time and temperature. Strikingly, Pb and Cd are only intermixed at the PbSe/CdSe interface. We propose a plausible model for the cation exchange based on a layer-by-layer replacement of Pb 2+ by Cd 2+ enabled by a vacancy-assisted cation migration mechanism.
Quasi-Seeded Growth of Ligand-Tailored PbSe Nanocrystals through Cation-Exchange-Mediated Nucleation
Angewandte Chemie International Edition, 2008
The solution-phase synthesis of inorganic nanocrystals (NCs) represents an efficient route towards a wide variety of compositions and morphologies at the nanoscale. A number of synthetic strategies are based on long-known or newly discovered phenomena in NC formation. The LaMer model, for example, introduced the idea of temporally separated nucleation and growth [1] and seeded growth. Typically, balancing the nucleation and growth rates represents one of the major challenges in the colloidal synthesis of monodisperse NCs, as these processes have different activation energies and reaction orders. Balancing them in a one-pot reaction has proven to be sometimes very difficult (for example, in the case of GaAs).
Tuning light emission of PbS nanocrystals from infrared to visible range by cation exchange
Science and Technology of Advanced Materials, 2015
Colloidal semiconductor nanocrystals, with intense and sharp-line emission between red and near-infrared spectral regions, are of great interest for optoelectronic and bio-imaging applications. The growth of an inorganic passivation layer on nanocrystal surfaces is a common strategy to improve their chemical and optical stability and their photoluminescence quantum yield. In particular, cation exchange is a suitable approach for shell growth at the expense of the nanocrystal core size. Here, the cation exchange process is used to promote the formation of a CdS passivation layer on the surface of very small PbS nanocrystals (2.3 nm in diameter), blue shifting their optical spectra and yielding luminescent and stable nanostructures emitting in the range of 700-850 nm. Structural, morphological and compositional investigation confirms the nanocrystal size contraction after the cation-exchange process, while the PbS rock-salt crystalline phase is retained. Absorption and photoluminescence spectroscopy demonstrate the growth of a passivation layer with a decrease of the PbS core size, as inferred by the blue-shift of the excitonic peaks. The surface passivation strongly increases the photoluminescence intensity and the excited state lifetime. In addition, the nanocrystals reveal increased stability against oxidation over time. Thanks to their absorption and emission spectral range and the slow recombination dynamics, such highly luminescent nano-objects can find interesting applications in sensitized photovoltaic cells and light-emitting devices.
Selective Facet Reactivity during Cation Exchange in Cadmium Sulfide Nanorods
Journal of the American Chemical Society, 2009
The partial transformation of ionic nanocrystals through cation exchange has been used to synthesize nanocrystal heterostructures. We demonstrate that the selectivity for cation exchange to take place at different facets of the nanocrystal plays an important role in determining the resulting morphology of the binary heterostructure. In the case of copper(I) (Cu +) cation exchange in cadmium sulfide (CdS) nanorods, the reaction starts preferentially at the ends of the nanorods such that copper sulfide (Cu 2 S) grows inward from either end. The resulting morphology is very different from the striped pattern obtained in our previous studies of silver(I) (Ag +) exchange in CdS nanorods where nonselective nucleation of silver sulfide (Ag 2 S) occurs (
Journal of the American Chemical Society, 2012
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 dotin-rod geometries obtained, while the hole states remain largely uncoupled.
Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene
Scientific Reports
Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in devices such as photodetectors and solar cells. Such devices typically employ electronic materials in which charge carrier concentrations are manipulated through ''doping;'' however, persistent electronic doping of these NCs remains a challenge. Here, we demonstrate that heavily doped n-type PbSe and PbS NCs can be realized utilizing ground-state electron transfer from cobaltocene. This allows injecting up to eight electrons per NC into the band-edge state and maintaining the doping level for at least a month at room temperature. Doping is confirmed by inter-and intra-band optical absorption, as well as by carrier dynamics. Finally, FET measurements of doped NC films and the demonstration of a p-n diode provide additional evidence that the developed doping procedure allows for persistent incorporation of electrons into the quantum-confined NC states.
A Simple Way To Prepare PbS Nanocrystals with Morphology Tuning at Room Temperature
The Journal of Physical Chemistry B, 2006
A simple way to synthesize PbS nanocrystals with the ability to tune their morphology at room temperature is reported. The preparation utilizes an amine-catalyzed decomposition of a precursor and the amine was found to play dual roles as both the catalyst and the capping agent. Spherical PbS nanocrystals of diameters 5 to 10 nm were obtained when long chain alkylamines were used in the pot. When difunctional ethylenediamine was used instead, exclusively PbS dendrites can be isolated from the same precursor at room temperature. Uniform six-and four-armed dendrites are observed, with regular branches of ∼20 nm in diameter growing in a parallel order. In a further step, morphology tuning of the dendrites to induce 1D growth into nanorods is achievable through the addition of a trace amount of stronger capping dodecanethiol molecules. Thus, PbS nanorods with aspect ratios of ∼20 to 30 could be successfully obtained and illustrated. A possible formation mechanism is discussed and the initial step of the reaction mechanism was modeled with DFT calculations as a nucleophilic attack.
In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals
Nature Materials, 2016
Oriented attachment of PbSe nanocubes can result in the formation of two-dimensional (2D) superstructures with long-range nanoscale and atomic order 1,2. This questions the applicability of classic models in which the superlattice grows by first forming a nucleus, followed by sequential irreversible attachment of nanocrystals 3,4 , as one misaligned attachment would disrupt the 2D order beyond repair. Here, we demonstrate the formation mechanism of 2D PbSe superstructures with square geometry by using in situ grazing-incidence X-ray scattering (small angle and wide angle), ex situ electron microscopy, and Monte Carlo simulations. We observed nanocrystal adsorption at the liquid/gas interface, followed by the formation of a hexagonal nanocrystal monolayer. The hexagonal geometry transforms gradually through a pseudo-hexagonal phase into a phase with square order, driven by attractive interactions between the {100} planes perpendicular to the liquid substrate, which maximize facetto-facet overlap. The nanocrystals then attach atomically via a necking process, resulting in 2D square superlattices. Oriented atomic attachment of colloidal nanocrystals (NCs), that is, the formation of a single crystal by atomic connection of smaller crystals, is an important process in geology 5-8 , and recently gained much attention as a preparation tool in semiconductor nanoscience 9,10. We reported a method to prepare 2D atomically coherent PbSe superlattices, starting from a suspension of PbSe NCs 1,2. The NCs have the shape of a truncated cube, consistent with the rock salt crystal structure of PbSe (see Supplementary Fig. 1). A suspension of these NCs is cast onto a surface of an immiscible liquid, ethylene glycol, and the solvent is evaporated at room temperature. During the evaporation, extended sheets are formed with a thickness of one NC monolayer 1. The 2D structure shows a nanoscale geometry with square periodicity with, to some extent, also atomic coherency. In this so-called square geometry, all NCs are directed with a 100 axis perpendicular to the 2D plane, and are laterally connected via the in-plane {100} facets. This means that two out of six {100} facets,